[llvm] [AMDGPU] Create AMDGPUMnemonicAlias tablegen class (PR #89288)

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

LGTM. I have a slight preference for using `let ... in ...` throughout instead of `def ... { let ... }`, because it tends to save a line, but I certainly won't insist.

https://github.com/llvm/llvm-project/pull/89288