[PowerPC] Fix combine step for shufflevector.

The combine step for shufflevector will sometimes replace undef in the mask
with a defined value. This can cause an infinite loop in some cases as another
combine will then put the undef back in the mask.

This patch fixes the issue so that undefs are not replaced when doing a combine.

Reviewed By: ZarkoCA, amyk, quinnp, saghir

Differential Revision: https://reviews.llvm.org/D127439