[llvm] [InstCombine] Fix i1 ssub.sat compare folding (PR #173742)

[Justin Lebar via llvm-commits]

jlebar wrote:

I haven't been able to find or recreate the failure that led me to this patch, something got lost in translation, but I'm not sure what.

I'm happy to add the `!Ty->isIntegerTy(1)` guard here in instcombine, if you're OK with that being untested.  Alternatively we could add an assert, but given how rare this is, it seems likely nobody will even notice the assert if it's hit.

WDYT?

Thank you for all the reviews.

https://github.com/llvm/llvm-project/pull/173742