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

[Han Shen via llvm-commits]

shenh10 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.

Yes, I have deleted a few confusions about how to understand your comments. Don't worry, I got it. Comments left are issues that still exist.

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