[llvm] [NVPTX] Fix code generation for `trap-unreachable`. (PR #67478)

[Artem Belevich via llvm-commits]

Artem-B wrote:

We've removed `nvptx-exit-on-unreachable` knob which used to disable generation of `exit`.

https://github.com/llvm/llvm-project/pull/72641 tries to disable it for some unreachable branches, which I'm not convinced we should do, but we do not seem to have an escape hatch option any more, as an alternative.

https://github.com/llvm/llvm-project/pull/67478