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

[Craig Topper via llvm-commits]

topperc wrote:

> 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.

It looks like the majority of the comments were from before the switch to visitMUL.

I'm still concerned about the maintainability by non-RISCV developers who don't care about VP. I'd like to hear from someone like @RKSimon.

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