[llvm] [SandboxVec][Scheduler][DAG] Fix the update of DGNode on setOperand (PR #214110)

[via llvm-commits]

github-actions[bot] wrote:

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

* 139300 tests passed
* 3641 tests skipped
* 1 test failed

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

### LLVM-Unit
<details>
<summary>LLVM-Unit.Transforms/Vectorize/SandboxVectorizer/_/SandboxVectorizerTests_exe/SchedulerTest/MaintainUnscheduledSuccsOnUseSet</summary>

```
Script:
--
C:\_work\llvm-project\llvm-project\build\unittests\Transforms\Vectorize\SandboxVectorizer\.\SandboxVectorizerTests.exe --gtest_filter=SchedulerTest.MaintainUnscheduledSuccsOnUseSet
--
C:\_work\llvm-project\llvm-project\llvm\unittests\Transforms\Vectorize\SandboxVectorizer\SchedulerTest.cpp:1176
Expected equality of these values:
  N1->getNumUnscheduledPreds()
    Which is: 1
  0u
    Which is: 0

C:\_work\llvm-project\llvm-project\llvm\unittests\Transforms\Vectorize\SandboxVectorizer\SchedulerTest.cpp:1185
Expected equality of these values:
  N1->getNumUnscheduledPreds()
    Which is: 1
  0u
    Which is: 0

C:\_work\llvm-project\llvm-project\llvm\unittests\Transforms\Vectorize\SandboxVectorizer\SchedulerTest.cpp:1194
Expected equality of these values:
  N1->getNumUnscheduledPreds()
    Which is: 1
  0u
    Which is: 0

C:\_work\llvm-project\llvm-project\llvm\unittests\Transforms\Vectorize\SandboxVectorizer\SchedulerTest.cpp:1203
Expected equality of these values:
  N1->getNumUnscheduledPreds()
    Which is: 1
  0u
    Which is: 0


```
</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/214110