[GlobalISel] Make use of KnownBits::computeForAddSub

Summary:
This is mostly NFC. computeForAddSub may give more precise results in
some cases, but that doesn't seem to affect any existing GlobalISel
tests.

Subscribers: rovka, hiraditya, volkan, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73431