vkd3d-shader: Use iterators everywhere, part 8/M.
More steps towards replacing the vsir instruction array.
As mentioned in !1675 (comment 112544) I found that every time we need to insert instructions before other instructions we also want to move the iterator to the first of the newly added instructions, thus I introduce vsir_program_iterator_insert_before_and_move().
If we ever want to do that and not move the iterator I guess we can introduce a plain vsir_program_iterator_insert_before(), but I didn't find any instance of that.