[InstCombine] refactor add narrowing folds; NFCI

The test diffs are all cosmetic due to the change in
value naming, but I'm including that to show that the
new code does perform these folds rather than something
else in instcombine.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342278 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed