[X86][SSE] Improve combineLogicBlendIntoPBLENDV to use general masks.

Currently combineLogicBlendIntoPBLENDV can only match ASHR to detect sign splatting of a bit mask, this patch generalises this to use computeNumSignBits instead.

This is a first step in several things we can do to improve PBLENDV support:

 * Better matching of X86ISD::ANDNP patterns.
 * Handle floating point cases.
 * Better vector and bitcast support in computeNumSignBits.
 * Recognise that PBLENDV only uses the sign bit of the mask, we should be able strip away sign splats (ASHR, PCMPGT isNeg tests etc.).

Differential Revision: https://reviews.llvm.org/D32953

llvm-svn: 302424