[llvm] [DAG] add (~a | x) & (a | y) -> (a & (x ^ y)) ^y for foldMaskedMerge (PR #144342)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Please can you pull out the new tests into a pre-commit patch against current trunk codgen? I'll get that committed and then you can update this patch to show the codegen changes

https://github.com/llvm/llvm-project/pull/144342