[SelectionDAG] ComputeNumSignBits - support constant pool values from target

[SelectionDAG] ComputeNumSignBits - support constant pool values from target

As I mentioned on D61887 we don't get many hits on ComputeNumSignBits as we did on computeKnownBits.

The case we do get is interesting though - it allows us to use the 'ConditionalNegate' combine in combineLogicBlendIntoPBLENDV to remove a select.

It comes too late for SSE41 (BLENDV) cases, but SSE2 tests can hit it now. We should probably try to make use of this for SSE41+ targets as well - avoiding variable blends is usually a good idea. I'll investigate as a followup.

Differential Revision: https://reviews.llvm.org/D62777

llvm-svn: 362486