[llvm] [InstCombine] Fix miscompilation caused by #90436 (PR #91133)

[Nikita Popov via llvm-commits]

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

LGTM, but can you add the tests to icmp-of-trunc-ext.ll instead of a separate file?

https://github.com/llvm/llvm-project/pull/91133