[llvm] [DAGCombiner] Port custom DAG combine for `rem` from NVPTX (PR #210344)

[Simon Pilgrim via llvm-commits]

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RKSimon wrote:

once you strip away everything that visitREM already provides performREMCombine is barely 10 LOC - just embed it here directly?

https://github.com/llvm/llvm-project/pull/210344