[llvm] [ConstantFolding] Handle roundeven libcalls (PR #170692)

[Nikita Popov via llvm-commits]

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

LGTM. Given that roundeven gets folded to the intrinsic by SLC, it's questionable how useful this really is, but I guess this makes sense for consistency if nothing else.

https://github.com/llvm/llvm-project/pull/170692