Fix the PPC subsumes-predicate check

For one predicate to subsume another, they must both check the same condition
register. Failure to check this prerequisite was causing miscompiles.

Fixes PR18003.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197089 91177308-0d34-0410-b5e6-96231b3b80d8