[llvm] [SelectionDAG] Handle roundeven libcall in visitCall (PR #170690)

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valadaptive wrote:

I noticed this when working on https://github.com/llvm/llvm-project/pull/170018 (see [these tests](https://github.com/llvm/llvm-project/pull/170018/files#diff-17b3a9745335b36c31ff84fe4a7cf502b0993cdafc7157e82eb4fa1e73046153R479-R542)). It looks like roundeven is already handled in SLC, but not getting converted to an intrinsic in those regression tests. I'm not sure if I just need to enable more optimization passes in the tests or something, but supporting it in SelectionDAG ([and hasOptimizedCodegen](https://github.com/llvm/llvm-project/pull/170689)) seems to be necessary.

https://github.com/llvm/llvm-project/pull/170690