[llvm] [DAGCombiner][RISCV] Optimize (zext nneg (truncate X)) if X has known sign bits. (PR #82227)

[Nikita Popov via llvm-commits]

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

LGTM, but can you please pre-commit the new tests?

https://github.com/llvm/llvm-project/pull/82227