[X86] Flip isShuffleEquivalent argument order to match isTargetShuffleEquivalent

A while ago, we converted isShuffleEquivalent/isTargetShuffleEquivalent to both use IsElementEquivalent internally.

This allows us to make the shuffle args optional like isTargetShuffleEquivalent and update foldShuffleOfHorizOp to use isShuffleEquivalent (which it should as its using a ISD::VECTOR_SHUFFLE mask).