Move PPC getSwappedPredicate for reuse

The getSwappedPredicate function can be used in other places (such as in
improvements to the PPCCTRLoops pass). Instead of trapping it as a static
function in PPCInstrInfo, move it into PPCPredicates with other
predicate-related things.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179926 91177308-0d34-0410-b5e6-96231b3b80d8