[InstCombine] Fold more intrinsics over selects

Move this handling to a centralized place and extend it to handle
saturating add/sub intrinsics.

I originally wanted to make this fully generic rather than
whitelist based, because this is legal and likely profitable for all
speculatable intrinsics. The caveat is that for vector selects,
the intrinsic can't perform cross-lane operations like a shuffle
or reduction, which we don't really expose as a generic property
right now. So for now I'm just extending the list.