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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Some of our custom expressions are not variadic

Or and max are, so what custom expressions aren't variadic? 


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