[X86][AVX] Improved VPERMILPS variable shuffle mask decoding.

Added support for decoding VPERMILPS variable shuffle masks that aren't in the constant pool.

Added target shuffle mask decoding for SCALAR_TO_VECTOR+VZEXT_MOVL cases - these can happen for v2i64 constant re-materialization

Followup to D17681

llvm-svn: 262784