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

[Mikael Holmen via llvm-commits]

mikaelholmen 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.

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