[GlobalISel] Add generic opcodes for saturating add/subtract

Summary:
Add new generic MIR opcodes G_SADDSAT etc. Add support in IRTranslator
for translating the saturating add/subtract intrinsics to the new
opcodes.

Reviewers: aemerson, dsanders, paquette, arsenm

Subscribers: jvesely, wdng, nhaehnle, rovka, hiraditya, volkan, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76600