[clang] [clang][CFG] Fix compound assignment evaluation order (PR #212115)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 90527 tests passed
* 1713 tests skipped
* 1 test failed

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

### Clang
<details>
<summary>Clang.Sema/warn-unreachable.c</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang -cc1 -internal-isystem /home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/24/include -nostdsysteminc -fsyntax-only -verify -fblocks -Wunreachable-code-aggressive -Wno-unused-value -Wno-covered-switch-default -I /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Sema/Inputs /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Sema/warn-unreachable.c
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang -cc1 -internal-isystem /home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/24/include -nostdsysteminc -fsyntax-only -verify -fblocks -Wunreachable-code-aggressive -Wno-unused-value -Wno-covered-switch-default -I /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Sema/Inputs /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Sema/warn-unreachable.c
# .---command stderr------------
# | error: 'expected-warning' diagnostics expected but not seen: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Sema/warn-unreachable.c Line 87: will never be executed
# | error: 'expected-warning' diagnostics seen but not expected: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Sema/warn-unreachable.c Line 86: code will never be executed
# | 2 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

```
</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/212115