[PowerPC] Add option to disable perfect shuffle

Perfect shuffle was introduced into PowerPC backend years ago, and only
available in big-endian subtargets. This optimization has good effects
in simple cases, but brings serious negative impact in large programs
with many shuffle instructions sharing the same mask.

Here introduces a temporary backend hidden option to control it until we
implemented better way to fix the gap in vectorshuffle decomposition.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D120072