[llvm] [NVPTX] Add support for atomic add for bf16 type (PR #89586)

[David Majnemer via llvm-commits]

majnemer wrote:

I think it is OK for us not to do anything special for `noftz`.

It is a bit tricky to fix it post-hoc because of cases like:
```
  x = 0
  x += 0x1.02p-126
  x += -0x1p-126
```
  
Here, none of the inputs were subnormal but `x` ends up as `0x1p-133` which is subnormal.

Also, LLVM semantics allow us to return subnormal results instead of flushing.

LGTM.

https://github.com/llvm/llvm-project/pull/89586