[PowerPC] Fallback to base's implementation of shouldExpandAtomicCmpXchgInIR...

[PowerPC] Fallback to base's implementation of shouldExpandAtomicCmpXchgInIR and shouldExpandAtomicCmpXchgInIR

If we can't decide `shouldExpandAtomicCmpXchgInIR` or `shouldExpandAtomicCmpXchgInIR` in PPC's implementation after https://reviews.llvm.org/rGb9c3941cd61de1e1b9e4f3311ddfa92394475f4b, resort to base's implementation.

This fixes internal build of OpenMP which uses atomic operations on float.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D106234