[InstCombine] Use uadd.sat and usub.sat for canonicalization

Start using the uadd.sat and usub.sat intrinsics for the existing
canonicalizations. These intrinsics should optimize better than
expanded IR, have better handling in the X86 backend and should
be no worse than expanded IR in other backends, as far as we know.

rL357012 already introduced use of uadd.sat for the add+umin pattern.

Differential Revision: https://reviews.llvm.org/D58872

llvm-svn: 357103