[PowerPC] Don't always consider P8Altivec-only masks in LowerVECTOR_SHUFFLE

[PowerPC] Don't always consider P8Altivec-only masks in LowerVECTOR_SHUFFLE

LowerVECTOR_SHUFFLE needs to decide whether to pass a vector shuffle off to the
TableGen-generated matching code, and it does this by testing the same
predicates used by the TableGen files. Unfortunately, when we added new
P8Altivec-only predicates, we started universally testing them in
LowerVECTOR_SHUFFLE, and if then matched when targeting a system prior to a P8,
we'd end up with a selection failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246675 91177308-0d34-0410-b5e6-96231b3b80d8