[PowerPC] Implement hasBitPreservingFPLogic for types that can be supported

This is the PPC-specific non-controversial part of
https://reviews.llvm.org/D44548 that simply enables this combine for PPC
since PPC has these instructions.
This commit will allow the target-independent portion to be truly target
independent.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344077 91177308-0d34-0410-b5e6-96231b3b80d8