[llvm] [DAG] Convert foldMaskedMerge to SDPatternMatch to match (m & x) | (~m & y) (PR #143855)

[Iris Shi via llvm-commits]

https://github.com/el-ev approved this pull request.

LGTM but please wait until the commutativity discussion is complete.

https://github.com/llvm/llvm-project/pull/143855