Optimize away unifications with dead variables.

modes.m:
	Optimize away unifications with dead variables.
	(The code generator already does that, but by
	that time we have already allocated unnecessary
	stack slots for them.)

code_exprn.m:
	Avoid generating sequences such as `r2 = r3; r3 = r2;'
	when a register already contains the correct value.
	(This change was in fact at least as much Tom's work as mine.)

store_alloc.m:
	For disjunctions, we only want to allocate registers
	for the variables that are output by the disjunction.
	The inputs should go in framevars, not in registers.
	This avoids much of the register-shuffling
	in the code generated for e.g. list__member(out, in).
	(This change was in fact at least as much Tom's work as mine.)

compiler/code_exprn.m

@@ -842,6 +842,20 @@ code_exprn__place_var_2(cached(Exprn0), Var, Lval, Code) -->
 		{ ExprnCode = node([
 			assign(Lval, Exprn) - Comment
 		]) }
+	;
+		% if the variable already has its value stored in the
+		% right place, we don't need to generated any code
+		{ Exprn0 = var(Var1) },
+		code_exprn__get_vars(Vars0),
+		{ map__search(Vars0, Var1, Stat0) },
+		{ Stat0 = evaled(VarRvals) },
+		{ set__member(lval(Lval), VarRvals) }
+	->
+		{ ClearCode = empty },
+		{ ExprnCode = empty },
+		code_exprn__add_lval_reg_dependencies(Lval),
+		{ set__singleton_set(LvalSet, lval(Lval)) },
+		{ Stat = evaled(LvalSet) }
 	;
 			% move stuff out of the way
 		code_exprn__clear_lval(Lval, Exprn0, Exprn1, ClearCode),
@@ -855,6 +869,7 @@ code_exprn__place_var_2(cached(Exprn0), Var, Lval, Code) -->
 		(
 			{ Exprn2 = create(_, _, _) }
 		->
+			% XXX why is this necessary?
 			code_exprn__get_var_rvals(Var, Rvals7),
 			{ set__to_sorted_list(Rvals7, RvalList7) },
 			{ code_exprn__select_rval(RvalList7, Exprn3) },