[llvm] [SelectionDAG][Targets] Replace atomic_load_8/atomic_load_16 with atomic_load_*ext_8/atomic_load_*ext_16 where possible. (PR #137279)

[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/15028

<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
0.940 [60/18/1] Linking CXX executable tools/clang/unittests/Basic/BasicTests
1.065 [59/18/2] Linking CXX executable tools/clang/unittests/Parse/ParseTests
1.453 [58/18/3] Linking CXX executable tools/clang/unittests/Format/FormatTests
5.113 [57/18/4] Linking CXX executable tools/clang/unittests/Lex/LexTests
5.512 [56/18/5] Linking CXX executable tools/clang/unittests/libclang/libclangTests
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=1221.803110

```

</details>

https://github.com/llvm/llvm-project/pull/137279