[llvm] [NVPTX] Set boolean contents to zero-or-one (PR #108969)

[Justin Fargnoli via llvm-commits]

https://github.com/justinfargnoli commented:

Although I'm not opposed to this change for it's own sake, I'm not confident that it's the right fix for this bug. 

Based on my current understanding, the root cause of this issue is that `v2i16 setcc` isn't being recognized as an illegal operation and then legalized to `v2i16 sext(v2i1 setcc)`. 

If that's correct, then I think we'd want to ensure that `v2i16 setcc` is correctly legalized instead of trying to ensure that the DAG combiner to never generates a `v2i16 setcc`. 

https://github.com/llvm/llvm-project/pull/108969