Lets us reduce inserts by seeing which bits in the selector mask
indicates a particular source is used more than the other one, and
then just uses that as the base to be inserted into, cutting down
on overall insertion overhead.
In some cases, this can be quite drastic, like with:
vpblendw ymm0, ymm1, ymm2, 0b00000001
being cut down from 98 instructions to 14.
LC
a6e7fba433