[llvm] [DAG] FoldConstantArithmetic - add support for ISD::MULO/ADDO/SUBO constant folding (PR #228877)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Add the baseline tests as the first commit in the PR's commit chain so that we see the codegen diff in later commits - OK to rebase in this case (as you have no inline review comments yet)

https://github.com/llvm/llvm-project/pull/228877