[llvm] [VPlan] Fix header phi VPInstruction verification. NFC (PR #151472)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-win-fast` running on `as-builder-3` while building `llvm` at step 7 "test-build-unified-tree-check-llvm-unit".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/2/builds/30229

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

```
Step 7 (test-build-unified-tree-check-llvm-unit) failure: test (failure)
******************** TEST 'LLVM-Unit :: Transforms/Vectorize/./VectorizeTests.exe/50/52' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build\unittests\Transforms\Vectorize\.\VectorizeTests.exe-LLVM-Unit-6452-50-52.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=52 GTEST_SHARD_INDEX=50 C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build\unittests\Transforms\Vectorize\.\VectorizeTests.exe
--

Script:
--
C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build\unittests\Transforms\Vectorize\.\VectorizeTests.exe --gtest_filter=VPVerifierTest.NonHeaderPHIInHeader
--
C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\llvm-project\llvm\unittests\Transforms\Vectorize\VPlanVerifierTest.cpp(317): error: Expected equality of these values:
  "Found non-header PHI recipe in header VPBB: IR   <badref> = phi i32 \n"
  ::testing::internal::GetCapturedStderr().c_str()
    Which is: "Found non-header PHI recipe in header VPBB"


C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\llvm-project\llvm\unittests\Transforms\Vectorize\VPlanVerifierTest.cpp:317
Expected equality of these values:
  "Found non-header PHI recipe in header VPBB: IR   <badref> = phi i32 \n"
  ::testing::internal::GetCapturedStderr().c_str()
    Which is: "Found non-header PHI recipe in header VPBB"



********************


```

</details>

https://github.com/llvm/llvm-project/pull/151472