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

[Janek van Oirschot via llvm-commits]

JanekvO wrote:

> 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.

I was hoping to merge so I can add appropriate tests in #80855 that demonstrate the propagation that could occur for kernel descriptors. I'll take responsibility of reverting in case this change won't be needed anymore.

Thanks for the review!

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