[llvm] [AMDGPU] Eliminate likely-spurious execz checks via intrinsic argument (PR #123749)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

Rebased to resolve merge conflics in recently-updated tests:
```
  llvm/test/CodeGen/AMDGPU/GlobalISel/divergence-structurizer.mir
  llvm/test/CodeGen/AMDGPU/GlobalISel/regbankselect-mui-regbanklegalize.mir
  llvm/test/CodeGen/AMDGPU/GlobalISel/regbankselect-mui-regbankselect.mir
  llvm/test/CodeGen/AMDGPU/GlobalISel/regbankselect-mui.mir
  llvm/test/CodeGen/AMDGPU/machine-sink-lane-mask.mir
```

https://github.com/llvm/llvm-project/pull/123749