[X86][AVX] getTargetShuffleMask - don't decode VBROADCAST(EXTRACT_SUBVECTOR(X,0)) patterns.

getTargetShuffleMask is used by the various "SimplifyDemanded" folds so we can't assume that the bypassed extract_subvector can be safely simplified - getFauxShuffleMask performs a more general decode that allows us to more safely catch many of these cases so the impact is minimal.

(cherry picked from commit 5b5dc244)