This fixes Mantis bug #493.
compiler/common.m:
Don't apply the common_struct optimization if doing so could possibly
invalidate the annotations generated by mark_static_terms.m, which the
MLDS code generator relies on.
Move the tests for the applicability of the common_struct optimization
for both construction and deconstruction unifications next to each other.
Add XXXs about possible problems this code may have when applied to code
that does region based memory allocation.
tests/valid/bug493.m:
A simplified version of the Mantis test case.
tests/valid/Mmakefile:
tests/valid/Mercury.options:
Enable the new test case, and specify the options that originally caused
the bug to manifest itself.
18e222656f