In each 128-lane, if there is at least one index is demanded and not all

indices are demanded and this 128-lane is not the first 128-lane of the

legalized-vector, then this 128-lane needs a extracti128;

If in each 128-lane, there is at least one index is demanded, this 128-lane

needs a inserti128.

The following cases will help you build a better understanding:

Assume we insert several elements into a v8i32 vector in avx2,

Case#1: inserting into 1th index needs vpinsrd + inserti128

Case#2: inserting into 5th index needs extracti128 + vpinsrd +

inserti128

Case#3: inserting into 4,5,6,7 index needs 4*vpinsrd + inserti128.