Implement scalar and vector global data for the MLDS backend, modelled on

Estimated hours taken: 32
Branches: main

Implement scalar and vector global data for the MLDS backend, modelled on
the implementation of global data for the LLDS backend. Use scalar global
data to eliminate redundant copies of static memory cells. Use vector global
data to implement lookup switches, and to implement string switches more
efficiently.

The diff reduces the compiler executable's size by 3.3% by eliminating
duplicate copies of static cells. The diff can reduce the sizes of object files
not only through this reduction in the size of read-only data, but also through
reductions in the size of the needed relocation info: even in the absence of
duplicated cells, using one global variable that holds an array of all the
cells of the same type requires less relocation info than a whole bunch of
separate global variables each holding one cell. If C debugging is enabled,
we can also expect a significant reduction in the size of the debug information
stored in object files AND in executables, for the same reason. (This was the
original motivation for scalar static data on the LLDS backend; the large
amount of relocation information in object files, especially if Mercury
debugging was enabled, led to long link times.)

compiler/ml_global_data.m:
	Make the changes described above.

compiler/ml_lookup_switch.m:
	This new module implements lookup switches for the MLDS backend.
	For now, it implements only model_det and model_semi switches.

compiler/ml_switch_gen.m:
	Call the new module when appropriate.

	Do not require the switch generation methods that never generate
	definitions to return an empty list of definitions.

compiler/ml_backend.m:
	Add the new module.

compiler/notes/compiler_design.html:
	Mention the new module, and fix some documentation rot.

compiler/mlds.m:
	Extend the relevant types to allow the generated MLDS code to refer
	to both scalar and vector global data.

	Move a predicate that belongs here from ml_code_util.m.

	Rename a predicate to avoid ambiguity with its own return type.

	Give the functors of some types distinguishing prefixes.

compiler/ml_util.m:
	Replace some semidet predicates with functions returning bool,
	since the semidet predicates silently did the wrong thing on the new
	additions to the MLDS.

compiler/ml_code_gen.m:
	Ensure that we do not generate references to scalar and vector common
	cells on platforms which do not (yet) support them. At the moment,
	they are supported only when generating C.

	Put some code into a predicate of its own.

compiler/builtin_ops.m:
	Extend the type that represents array elements to allow them to be
	structures, which they are for vector globals.

compiler/ml_code_util.m:
	Add some new utility predicates and functions.

	Move some predicates that are now needed in more than one module here.

	Remove the predicates moved to mlds.m.

	Conform to the changes above.

compiler/ml_string_switch.m:
compiler/string_switch.m:
	Instead of two separate arrays, use one array of structures (a static
	vector), since they way, the string and the next slot indicator,
	which are accesses together, are next to each other and thus
	in the same cache block.

compiler/lookup_switch.m:
compiler/switch_util.m:
	Move several predicates from lookup_switch.m to switch_util.m,
	since now ml_lookup_switch.m needs them too. Parameterize the moved
	predicates as needed.

	Conform to the changes above.

compiler/llds.m:
	Add prefixes to some functor names to avoid ambiguities.

compiler/llds_out.m:
compiler/lookup_util.m:
compiler/mercury_compile.m:
	Minor style improvements.

compiler/global_data.m:
	Minor cleanups. Give names to some data types, and add prefixes to some
	field names.

	Conform to the changes above.

compiler/jumpopt.m:
	Minor style improvements.

	Conform to the changes above.

compiler/opt_debug.m:
	Fix some misleading variable names.

compiler/reassign.m:
	Factor out some duplicated code.

compiler/ll_pseudo_type_info.m:
compiler/ml_closure_gen.m:
compiler/ml_elim_nested.m:
compiler/ml_optimize.m:
compiler/ml_tag_switch.m:
compiler/ml_tailcall.m:
compiler/ml_unify_gen.m:
compiler/mlds_to_c.m:
compiler/mlds_to_gcc.m:
compiler/mlds_to_il.m:
compiler/mlds_to_java.m:
compiler/mlds_to_managed.m:
compiler/rtti_to_mlds.m:
compiler/stack_layout.m:
compiler/unify_gen.m:
	Conform to the changes above.

tests/hard_coded/lookup_switch_simple.{m,exp}:
tests/hard_coded/lookup_switch_simple_bitvec.{m,exp}:
tests/hard_coded/lookup_switch_simple_non.{m,exp}:
tests/hard_coded/lookup_switch_simple_opt.{m,exp}:
	New test cases to exercise the new functionality.

tests/hard_coded/Mmakefile:
tests/hard_coded/Mercury.options:
	Enable the new tests.

compiler/opt_debug.m

@@ -792,9 +792,9 @@ dump_instr(MaybeProcLabel, PrintComments, Instr) = Str :-
         Instr = goto(CodeAddr),
         Str = "goto " ++ dump_code_addr(MaybeProcLabel, CodeAddr)
     ;
-        Instr = computed_goto(Rval, Labels),
+        Instr = computed_goto(Rval, MaybeLabels),
         Str = "computed_goto " ++ dump_rval(MaybeProcLabel, Rval) ++ ":"
-            ++ dump_labels_or_not_reached(MaybeProcLabel, Labels)
+            ++ dump_labels_or_not_reached(MaybeProcLabel, MaybeLabels)
     ;
         Instr = arbitrary_c_code(AL, _, Code),
         Str = "arbitrary_c_code(" ++ dump_affects_liveness(AL) ++ "\n" ++