[llvm] AMDGPU/GlobalISel: Fix inst-selection of ballot (PR #109986)

[Jay Foad via llvm-commits]

jayfoad wrote:

For completeness llvm.amdgcn.ballot should be documented at https://llvm.org/docs/AMDGPUUsage.html#llvm-ir-intrinsics, and I think the documentation should say that result bits corresponding to inactive lanes are guaranteed to be zero.

https://github.com/llvm/llvm-project/pull/109986