[InstCombine] Canonicalize uadd.with.overflow to uadd.sat

This adds some patterns to transform uadd.with.overflow to uadd.sat
(with usub.with.overflow to usub.sat too). The patterns selects from
UINTMAX (or 0 for subs) depending on whether the operation overflowed.

Signed patterns are a little more involved (they can wrap in two
directions), but can be added here in a followup patch too.

Differential Revision: https://reviews.llvm.org/D69245