[X86] isVectorShiftByScalarCheap - vXi8...

[X86] isVectorShiftByScalarCheap - vXi8 select(shift(x,splat0),shift(x,splat1)) is better than shift(x,select(splat0,splat1))

Even though we don't have vXi8 vector shifts (apart from XOP), it is still better to prefer shift (or funnel-shift/rotate) by scalar where possible.

https://llvm.godbolt.org/z/6ss6ffTxv

Differential Revision: https://reviews.llvm.org/D116191