[X86] computeKnownBitsForTargetNode - add generic handling of PSHUFB

When PSHUFB is used as a LUT (for CTPOP, BITREVERSE etc.), its the source operand that is constant and the index operand the variable. As long as the indices don't set the MSB (which zeros the output element), then the common known bits from the source operand can be used directly, even though the shuffle mask isn't constant.

Further helps to improve CTPOP reduction codegen