[llvm] [AMDGPU][MC] Improve error message for missing dim operand (PR #96588)

[Diana Picus via llvm-commits]

rovka wrote:

This looks like an improvement over the status quo, but why did you choose to do this rather than replicate sp3's behavior? You should probably update [#47585](https://llvm.org/bz48241) to explain your thought process.

https://github.com/llvm/llvm-project/pull/96588