[InstCombine] Fix i1 ssub.sat compare folding (#173742)
For every type other than i1, ssub.sat x, y = 0 implies x == y. But
ssub.sat.i1 0, -1 = 0 (because the result of 1 saturates to 0).
The changes to instcombine are not strictly necessary. Instcombine
canonicalizes the ssub.sat.i1 before we arrive at these pattern-matches.
The real fix is in ValueTracking.
Nonetheless we agreed in review it makes sense to add these checks to
instcombine, even though they're currently unreachable:
https://github.com/llvm/llvm-project/pull/173742#issuecomment-3696631396
This was found by a fuzzer I'm working on!
