Codegen: [PPC] Handle weighted comparisons when inserting selects.

Only non-weighted predicates were handled in PPCInstrInfo::insertSelect. Handle
the weighted predicates as well.

This latent bug was triggered by r255398, because it added use of the
branch-weighted predicates.

While here, switch over an enum instead of an int to get the compiler to enforce
totality in the future.

llvm-svn: 257518