[llvm] [RISCV][GISEL] Legalize G_VSCALE (PR #85967)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Have you tried a negative factor for vscale? I got confused by: https://github.com/llvm/llvm-project/blob/b20360abeb3a80281dc082f1e093abd13cb1ee4c/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L3962 `buildVScale` takes an unsigned, which makes it hard to put negative scales in.

I think that we should not be able to put in negative scales right? This should be enforced my MachineVerifier.

https://github.com/llvm/llvm-project/pull/85967