This fixes the general case of Mantis bug 480.
compiler/cse_detection.m:
When deciding whether we want to pull common X = f(...) unifications
out of branched control structures, require only f'a args to be nonunique,
not the args of any other functors that X may be bound to.
compiler/switch_detection.m:
Obey the restrictions that cse_detection.m may impose.
tests/valid/bug480a.m:
A new test case for this bug fix.
tests/valid/Mmakefile:
Enable the new test case.
