[InstCombine] rearrange saturated add folds; NFC

This is no-functional-change-intended, but that was also
true when it was part of rL354276, and I managed to lose
2 predicates for the fold with constant...causing much bot
distress. So this time I'm adding a couple of negative tests
to avoid that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354384 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed