BUILD_VECTOR is lowed to as a immediate integer if all elements are constant.
Otherwise treat it as inserting each element to a undefined vector.
VECTOR_SHUFFLE according to its indexes would be lowered to byte swap, halfword swap, halfword packing
and word packing. Or as vselect, which is matched to bpick.
INSERT_VECTOR_ELT is simply lowed to the customized node INSERTB[H][W], and
matched it to insert bytes, halfword packing and word packing instructions.