[llvm] AMDGPU: Ensure both wavesize features are not set (PR #159234)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> It's useful for assembling a single source file containing wave32 and wave64 code. This is common in graphics where you can have a single pipeline containing two shaders, like a vertex shader using wave32 and a pixel shader using wave64.

To actually support this you would need to have MCSubtargetInfo specified per function, and not globally. Just turning on both modes globally is not the same thing 



https://github.com/llvm/llvm-project/pull/159234