[llvm] [NVPTX] Add support for atomic add for f16 type (PR #84295)

[Adrian Kuegel via llvm-commits]

akuegel wrote:

By now I have created a reproducer for what caused the revert. If I adjust the test slightly, and change the line with %r1:

```
%r1 = atomicrmw fadd ptr %dp0, half 1.0 seq_cst, align 2
```
The codegen for this becomes:

```
ld.param.u64 %rd1, [test_param_0]; 
atom.add.noftz.f16 %rs1, [%rd1], 0x3C00;
```

And this fails verification with this error:

```
Arguments mismatch for instruction 'atom'
```

I lack the knowledge about PTX to know what is wrong with that. Will try to figure it out.

https://github.com/llvm/llvm-project/pull/84295