[llvm] [RISCV] Match mul_vl(v, (add_vl v1, splat 1)) to vmadd_vl (PR #71495)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

Can we do this in DAGCombine? We already have a large MatcherTable now.
And scalable tests are needed too.

https://github.com/llvm/llvm-project/pull/71495