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

[via llvm-commits]

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def3r wrote:

This is what NVPTX ISel version had. The NVPTX tests explicitly test it not to combine at -O0 in [divrem-combine.ll](https://github.com/llvm/llvm-project/blob/7c1b00b38d7fb9eecfd345c51f9e9509e6b039ad/llvm/test/CodeGen/NVPTX/divrem-combine.ll#L16-L23).

Where else should the opt level be checked, or should the test be updated?

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