[llvm] [IR][AtomicExpand] Add elementwise modifier to atomicrmw; automatically expand for all targets (PR #189517)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 132912 tests passed
* 3042 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### LLVM-Unit
<details>
<summary>LLVM-Unit.IR/_/IRTests_exe/VerifierTest/AtomicRMW</summary>

```
Script:
--
C:\_work\llvm-project\llvm-project\build\unittests\IR\.\IRTests.exe --gtest_filter=VerifierTest.AtomicRMW
--
C:\_work\llvm-project\llvm-project\llvm\unittests\IR\VerifierTest.cpp:381
Value of: verifyFunction(*F, &ErrorOS)
  Actual: false
Expected: true

C:\_work\llvm-project\llvm-project\llvm\unittests\IR\VerifierTest.cpp:384
Value of: StringRef(Error).starts_with( "atomicrmw fadd operand must have floating-point or " "fixed vector of floating-point type!")
  Actual: false
Expected: true


```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/189517