[llvm] [GitHub][RISCV] Restrict backend:RISC-V labelling to Clang and LLVM (PR #192429)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> I also agree for VPlan/SLP we already have the vectorizers label so I don't think we need to specifically add the backend tags for it.

I guess most folks here already subscribed to VPlan/SLP so this holds true. But for other transformations tests I think it's still useful to get notified for updates on RISC-V-specific tests for people to take a look just in case.

https://github.com/llvm/llvm-project/pull/192429