[X86][SSE] lowerShuffleAsDecomposedShuffleBlend - support decomposed unpacks for...

[X86][SSE] lowerShuffleAsDecomposedShuffleBlend - support decomposed unpacks for some vXi8/vXi16 cases

Follow up to D86429 to handle the remaining regressions.

This patch generalizes lowerShuffleAsDecomposedShuffleBlend to lowerShuffleAsDecomposedShuffleMerge, and attempts to use an UNPCKL shuffle mask instead of a blend for the cases where the inputs are coming from alternating vXi8/vXi16 sources. Technically they don't have to be alternating (just as long as they can fit into a lower lane half for the unpack) but I didn't find as many general cases and it needed a lot more of the function to be altered.

For vXi32/vXi64 cases this could still be beneficial but in most cases the existing permute+blend approach was better.

Differential Revision: https://reviews.llvm.org/D87405