[llvm] [LLVM][NVPTX]Add BF16 vector instruction and fix lowering rules (PR #69415)

[Artem Belevich via llvm-commits]

Artem-B wrote:

@shenh10 I've got a handful of comment notifications in my email that I don't see here on the pull request itself. I wonder if they were removed, or I've messed up the pull request view somehow in my browser. I'm still discovering new (for me) GitHub's user interface quirks.

https://github.com/llvm/llvm-project/pull/69415