[llvm-branch-commits] RISCVAsmParser: Reject call foo at invalid (PR #135509)

[Fangrui Song via llvm-branch-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/135509

... instead of silently parsing and ignoring it without leaving an error
message.

While here, remove an unreachable `@plt`.