[x86] refactor combineAddOrSubToADCOrSBB(); NFCI

The comments were wrong, and this is not an obvious transform.
This hopefully makes it clearer that we're missing the commuted
patterns for adds. It's less clear that this is actually a good
transform for all micro-arch.

This is prep work for trying to clean up the current adc/sbb 
codegen because it's definitely not happening optimally.

llvm-svn: 296918