[PowerPC] add a peephole to remove redundant swap instructions after vector splats...

[PowerPC] add a peephole to remove redundant swap instructions after vector splats on P8

Vector store on P8 little endian will have swap instruction added before
the store in PPCISelLowring. If the vector is generated by splat, the
swap instruction can be eliminated.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D139691