[llvm] [AMDGPU][NFC] Rename AMDGPUVariadicMCExpr to AMDGPUMCExpr. (PR #96618)

[Janek van Oirschot via llvm-commits]

JanekvO wrote:

> > Some of our custom expressions are not variadic
> 
> Or and max are, so what custom expressions aren't variadic?

I've added `Occupancy`, `AlignTo`, `ExtraSGPR`, and `TotalNumVGPRs` to that class after having (initially, exclusively) used the AMDGPUMCExpr for `max` and `or` rendering the variadic attribute incorrect. Let me know if there's a preference to have those split off.

https://github.com/llvm/llvm-project/pull/96618