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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

It's probably a net win to treat the aliases the same way as the base instruction. Consistency is usually best 

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