Automerge: Improve file patterns for some backends.

I noticed that one of my PRs, #147624, did not get automatically
labelled with `backend:RISC-V` when it touched a test under
`llvm/test/CodeGen/RISCV`. This seems to be because the pattern only
looks for files named `*RISCV*` or `*riscv* and not necessarily files
in a directory named like that. To fix that, let's make RISC-V use
a more accepting pattern, similar to AMDGPU, and do the same for two
other backends.

Reviewers: tstellar, arsenm

Reviewed By: arsenm

Pull Request: llvm/llvm-project#147633

Author: pcc