[llvm] [AMDGPU] Fix whole-wave function prologue to set EXEC to -1 (PR #207781)

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

> @aobolensk These fail with EXPENSIVE_CHECKS:
> 
> ```
> Failed Tests (2):
>   LLVM :: CodeGen/AMDGPU/whole-wave-functions-pei.mir
>   LLVM :: CodeGen/AMDGPU/whole-wave-functions.ll
> ```
> 
> Also seen in the build bot failure in the previous comment.

Thanks for reporting. I have created a new one that is supposed to fix that: https://github.com/llvm/llvm-project/pull/207924 

https://github.com/llvm/llvm-project/pull/207781