[llvm] [IR] Remove the AtomicMem*Inst helper classes (PR #138710)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `arc-builder` running on `arc-worker` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/3/builds/15554

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

```
Step 6 (test-build-unified-tree-check-all) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
PASS: lit :: shtest-not.py (81907 of 81917)
PASS: lit :: xunit-output.py (81908 of 81917)
PASS: lit :: shtest-external-shell-kill.py (81909 of 81917)
PASS: lit :: xfail-cl.py (81910 of 81917)
PASS: lit :: googletest-timeout.py (81911 of 81917)
PASS: lit :: selecting.py (81912 of 81917)
PASS: lit :: shtest-timeout.py (81913 of 81917)
PASS: lit :: max-time.py (81914 of 81917)
PASS: lit :: shtest-shell.py (81915 of 81917)
PASS: lit :: shtest-define.py (81916 of 81917)
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1670.579168

```

</details>

https://github.com/llvm/llvm-project/pull/138710