[X86] lowerShuffleAsDecomposedShuffleMerge(): if both inputs are...

[X86] lowerShuffleAsDecomposedShuffleMerge(): if both inputs are broadcastable/identities, canonicalize broadcasts as such

Split off from D108253.
Broadcast is simpler than any other shuffle we might produce
to do what we want to do here, so prefer it.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D108382