[llvm] [SCEV] Special case sext in isKnownNonZero (PR #77834)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM. We could also compute a better unsigned range for sext, but no need to block on that.

https://github.com/llvm/llvm-project/pull/77834