This fixes github issue #64.
compiler/cse_detection.m:
When pulling a unification X = f(Y1, ..., Yn) out of an arm of
a disjunction, switch or if-then-else, require the instantiation state
of X to be free of unique or mostly_unique components.
Put the requirements on X's inst into a single predicate.
Add a mechanism to make debugging similar issues easier.
tests/valid/cse_unique.m:
A suitably modified form of the test program on github.
tests/valid/Mmakefile:
Enable the new test case.
