[llvm] [NVPTX] Fixup ISel patterns for setcc of i8 extract (PR #151204)

[Justin Fargnoli via llvm-commits]

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

To confirm my understanding: previously, the expansion didn’t correctly sext the byte extracted by prmt, leading to incorrect code generation. This change fixes that behavior, so the test case “regressions” are actually corrections. If that’s accurate, then LGTM 👍

https://github.com/llvm/llvm-project/pull/151204