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

[Thorsten Schütt via llvm-commits]

tschuett 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.

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