[X86] Improve (vzmovl (insert_subvector)) combine to handle a bitcast between the vzmovl and insert

This combine tries shrink a vzmovl if its input is an
insert_subvector. This patch improves it to turn
(vzmovl (bitcast (insert_subvector))) into
(insert_subvector (vzmovl (bitcast))) potentially allowing the
bitcast to be folded with a load.