This is the last significant change suggested in PR37806:
...though there are several follow-ups suggested in the code comments in this patch to complete this transform.
It's possible that a binop feeding a select-shuffle has been eliminated by earlier transforms (or the code was just written like this in the 1st place), so we'll fail to match the patterns that have 2 binops from D48401, D48678, D48662, D48485.
In that case, we can try to materialize identity constants for the remaining binop to fill in the "ghost" lanes of the vector (where we just want to pass through the original values of the source operand).
I added comments to ConstantExpr::getBinOpIdentity() to show planned follow-ups. For now, we only handle the 5 commutative integer binops (add/mul/and/or/xor).