This fixes Mantis bug #512, whose symptom is a compiler abort in some
very specific circumstances. The regression test for the bug below
documents the long causal chain needed to tickle this abort.
compiler/simplify_goal_conj.m:
When deleting unreachable code, set the requanify flag, which also
causes instmap_deltas to be recomputed. The immediate cause of the bug
was an instmap_delta that should have been thus recomputed, but wasn't.
Do this in both places where simplification can delete dead code,
though the Mantis 512 test case involves only one.
tests/valid/bug512.m:
A regression test for this bug.
tests/valid/Mercury.options:
tests/valid/Mmakefile:
Enable the new test case, and run it with the flags that tickle
the bug it is guarding against.
