[llvm] Add Dead Block Elimination to NVVMReflect (PR #144171)

[Yonah Goldberg via llvm-commits]

YonahGoldberg wrote:

> However, nothing stops users from using `__nvvm_reflect()` wherever they want, in an arbitrary convoluted way that we may not always be able to const-eval at build-time.

This is fair. I think there's some argument to be made that we should always at least clean up the most common use case, which is code that looks like: `if (__nvvm_reflect(something)) { do something } else { do something else }`. That's what this PR adds. But I understand your point.

https://github.com/llvm/llvm-project/pull/144171