[PowerPC] Fix fast-isel when compare is split from branch

When the compare feeding a branch was in a different BB from the branch, we'd
try to "regenerate" the compare in the block with the branch, possibly trying
to make use of values not available there. Copy a page from AArch64's play book
here to fix the problem (at least in terms of correctness).

Fixes PR23640.

llvm-svn: 238097