Fix assert when decoding PSHUFB mask

The PSHUFB mask decode routine used to assert if the mask index was out of
range (<0 or greater than the size of the vector).  The problem is, we can
legitimately have a PSHUFB with a large index using intrinsics.  The
instruction only uses the least significant 4 bits.  This change removes the
assert and masks the index to match the instruction behaviour.