[X86][AVX] matchShuffleAsBlend - use isElementEquivalent to help match broadcast/repeated elements

Extend matchShuffleAsBlend to not only match against known in-place elements for BLEND shuffles, but use isElementEquivalent to determine if the shuffle mask's referenced element is the same as the in-place element.

This allows us to replace a number of insertps instructions with more general blendps instructions (better opportunities for commutation, concatenation etc.).