[clang] [llvm] [NVPTX] Cleanup and document nvvm.fabs intrinsics, adding f16 support (PR #135644)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-nvptx-nvidia-ubuntu` running on `as-builder-7` while building `clang,llvm` at step 6 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/180/builds/16474

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/NVPTX/reg-copy.ll' FAILED ********************
Exit Code: 255

Command Output (stderr):
--
/home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llc < /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/CodeGen/NVPTX/reg-copy.ll -mtriple=nvptx64 -mcpu=sm_35 | /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/FileCheck /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/CodeGen/NVPTX/reg-copy.ll # RUN: at line 1
+ /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llc -mtriple=nvptx64 -mcpu=sm_35
+ /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/FileCheck /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/CodeGen/NVPTX/reg-copy.ll
/home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llc < /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/llvm-project/llvm/test/CodeGen/NVPTX/reg-copy.ll -mtriple=nvptx64 -mcpu=sm_35 | /usr/local/cuda/bin/ptxas -arch=sm_60 -c - # RUN: at line 2
+ /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/bin/llc -mtriple=nvptx64 -mcpu=sm_35
+ /usr/local/cuda/bin/ptxas -arch=sm_60 -c -
ptxas /home/buildbot/worker/as-builder-7/ramdisk/llvm-nvptx-nvidia-ubuntu/build/lit-tmp-m297f2rq/tmpxft_00222081_00000000-0_stdin, line 10; fatal   : Parsing error near '.nvvm': syntax error
ptxas fatal   : Ptx assembly aborted due to errors

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/135644