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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@mshockwave Is working on #78654 which should provide pattern matching helpers like we have for IR transdforms and GlobalISel - it'd make sense if we can decide now how we can make VP work with such an approach.

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