[llvm] SLP/RISCV: add negative test for lrint (#55208) (PR #65611)

[Philip Reames via llvm-commits]

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

LGTM w/comment addressed.

Please add a positive test for rint in the same file so the difference is visible.

https://github.com/llvm/llvm-project/pull/65611