[llvm] [DAGCombiner][VP] Add DAGCombine for VP_MUL (PR #80105)

[Shao-Ce SUN via llvm-commits]

https://github.com/sunshaoce approved this pull request.

LGTM. Because this patch has already undergone thorough review, and a similar one (https://github.com/llvm/llvm-project/pull/79101) has already been merged. I think it can be merged if there are no objections from others.

https://github.com/llvm/llvm-project/pull/80105