[llvm] [VPlan] Impl VPlan-based pattern match for ExtendedRed and MulAccRed (NFCI) (PR #113903)

[David Green via llvm-commits]

davemgreen wrote:

This is another one that is behaving differently, I think due to subtleties about when one-use checks are beneficial. https://godbolt.org/z/fYW9Y5TqG. There is a third more awkward case with interleave groups I have not looked into much yet.

I will try and make those into test cases to ensure we have the test coverage for them. I will have to check again later if anything else remains that is behaving differently and hitting the assert.

https://github.com/llvm/llvm-project/pull/113903