[X86] getFauxShuffleMask - merge insertelement paths

Merge the INSERT_VECTOR_ELT/SCALAR_TO_VECTOR and PINSRW/PINSRB shuffle mask paths - they both do the same thing (find source vector + handle implicit zero extension). The PINSRW/PINSRB path also handled in the insertion of zero case which needed to be added to the general case as well.