[llvm] [AMDGPU] Cleanup hasUnwantedEffectsWhenEXECEmpty function (PR #70206)

[via llvm-commits]

ruiling wrote:

I don't think the change is ok. The function `hasUnwantedEffectsWhenEXECEmpty()` is used to query whether we can optimize away a `s_cbranch_exez`, that is whether a block can be always executed even the EXEC is zero, **and the block execution with zero EXEC should not cause any observable side-effect to the program**. In theory, I think any instruction write to SGPR could possibly cause sideffect to the program. Although in practice, we might not hit this, I am not sure about this. But I think we should revert this, and I think it is even better to say scalar instructions also have unwanted-side-effect-when-exec-zero. @nhaehnle @perlfu 

https://github.com/llvm/llvm-project/pull/70206