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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

The MaschineVerifier only forbids 0 IIRC. It is a common pattern sub(x, C) -> add(x, -C).

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