[llvm] [RISCV] Decompose LMUL > 1 reverses into LMUL * M1 vrgather.vv (PR #104574)

[Craig Topper via llvm-commits]

topperc wrote:

> That isn't reflected in the scheduling models, though, unless I don't understand how they work. The P670 scheduling models reports 1/2/4/8 scaling, and the P470 1/12/16/24. See [RISCVSchedSiFiveP600.td](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/RISCV/RISCVSchedSiFiveP600.td#L737) and the [P470 PR](https://github.com/llvm/llvm-project/pull/102155)

The scheduling model numbers came from llvm-exegesis, but we realized yesterday that llvm-exegesis wasn't controlling the input indices to produce the worst case.

https://github.com/llvm/llvm-project/pull/104574