[X86] isTargetShuffleEquivalent - attempt to match SM_SentinelZero shuffle...

[X86] isTargetShuffleEquivalent - attempt to match SM_SentinelZero shuffle mask elements using known bits

If the combined shuffle mask requires zero elements, we don't currently have much chance of matching them against the expected source vector. This patch uses the SelectionDAG::MaskedVectorIsZero wrapper to attempt to determine if the expected lement we want to use is already known to be zero.

I've also tightened up the ExpectedMask assertion to always be in range - we're never giving it a target shuffle mask that has sentinels at all - allowing to remove some of the confusing bounds checks.

This attempts to address some of the regressions uncovered by D129150 where we more aggressively fold shuffles as AND / 'clear' masks which results in more combined shuffles using SM_SentinelZero.

Differential Revision: https://reviews.llvm.org/D129207