[x86] improve CMOV codegen by pushing add into operands

This is not the transform direction we want in general,
but by the time we have a CMOV, we've already tried
everything else that could be better.
The transform increases the uses of the other add operand,
but that is safe according to Alive2:
https://alive2.llvm.org/ce/z/Yn6p-A

We could probably extend this to other binops (not just add).
This is the motivating pattern discussed in:
https://llvm.org/PR51069

The test with i8 shows a missed fold because there's a trunc
sitting in front of the add. That can be handled with a small
follow-up.

Differential Revision: https://reviews.llvm.org/D106607