[llvm] [RISCV] Incorporate scalar addends to extend vector multiply accumulate chains (PR #168660)

[Ryan Buchner via llvm-commits]

bababuck wrote:

> can this be solved by MachineCombiner as well?

I believe so. I tried to do this with my initial approach before getting direction from Philip. There was technically a path forward there, but it was a much messier change (having to match with the different `VPseudos`) compared with this DAGCombiner-level change.

https://github.com/llvm/llvm-project/pull/168660