[llvm] [AMDGPU] Add AMDGPU specific variadic operation MCExprs (PR #82022)

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh approved this pull request.

Already approving as this is good to me, just some minor style fixes needed.
Wait for @arsenm as well to approve.

Do you plan to merge this directly w/o any users, or merge it later alongside some code that uses those expressions?
I'd prefer to see some code that uses the expressions to be merged at around the same time, this avoids having dead code in the backend if the project gets dropped/changes direction/etc.

https://github.com/llvm/llvm-project/pull/82022