[llvm] [SandboxVec][BottomUpVec] Fix packing when PHIs are present (PR #124206)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clangd-ubuntu-tsan` running on `clangd-ubuntu-clang` while building `llvm` at step 6 "test-build-clangd-clangd-index-server-clangd-in...".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/134/builds/12303

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

```
Step 6 (test-build-clangd-clangd-index-server-clangd-in...) failure: test (failure)
******************** TEST 'Clangd :: signature-help-with-offsets.test' FAILED ********************
Exit Code: 66

Command Output (stderr):
--
RUN: at line 1: clangd -lit-test < /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/clang-tools-extra/clangd/test/signature-help-with-offsets.test | /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build/bin/FileCheck -strict-whitespace /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/clang-tools-extra/clangd/test/signature-help-with-offsets.test
+ clangd -lit-test
+ /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build/bin/FileCheck -strict-whitespace /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/clang-tools-extra/clangd/test/signature-help-with-offsets.test
WARNING: ThreadSanitizer: unexpected memory mapping 0x79ffff672000-0x79ffffb00000
FATAL: ThreadSanitizer: unexpectedly found incompatible memory layout.
FATAL: Please file a bug.
I[00:39:38.213] clangd version 20.0.0git (https://github.com/llvm/llvm-project.git d2234ca16310a9e9bd595561353556ea6ba0176f)
I[00:39:38.213] Features: linux+debug+tsan+grpc
I[00:39:38.213] PID: 402999
I[00:39:38.214] Working directory: /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/build/tools/clang/tools/extra/clangd/test
I[00:39:38.214] argv[0]: clangd
I[00:39:38.214] argv[1]: -lit-test
I[00:39:38.214] Starting LSP over stdin/stdout
V[00:39:38.214] <<< {
  "id": 0,
  "jsonrpc": "2.0",
  "method": "initialize",
  "params": {
    "capabilities": {
      "textDocument": {
        "signatureHelp": {
          "signatureInformation": {
            "parameterInformation": {
              "labelOffsetSupport": true
            }
          }
        }
      }
    },
    "processId": 123,
    "rootPath": "clangd",
    "trace": "off"
  }
}

I[00:39:38.214] <-- initialize(0)
I[00:39:38.215] --> reply:initialize(0) 1 ms
==================
WARNING: ThreadSanitizer: signal-unsafe call inside of a signal (pid=402999)
    #0 free <null> (clangd+0xe1d41f) (BuildId: c98973291005200dfdc9c01c971edb04f4b1d72c)
    #1 __call_tls_dtors <null> (libc.so.6+0x438b3) (BuildId: f7307432a8b162377e77a182b6cc2e53d771ec4b)
    #2 SignalHandler(int) /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/llvm/lib/Support/Unix/Signals.inc (clangd+0x105163f) (BuildId: c98973291005200dfdc9c01c971edb04f4b1d72c)
    #3 __tsan::CallUserSignalHandler(__tsan::ThreadState*, bool, bool, int, __sanitizer::__sanitizer_siginfo*, void*) tsan_interceptors_posix.cpp.o (clangd+0xe262d5) (BuildId: c98973291005200dfdc9c01c971edb04f4b1d72c)
    #4 llvm::raw_fd_ostream::write_impl(char const*, unsigned long) /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/llvm/lib/Support/raw_ostream.cpp:764:19 (clangd+0x1037842) (BuildId: c98973291005200dfdc9c01c971edb04f4b1d72c)
    #5 llvm::raw_ostream::flush_nonempty() /vol/worker/clangd-ubuntu-clang/clangd-ubuntu-tsan/llvm-project/llvm/lib/Support/raw_ostream.cpp:222:3 (clangd+0x1035d3b) (BuildId: c98973291005200dfdc9c01c971edb04f4b1d72c)
...

```

</details>

https://github.com/llvm/llvm-project/pull/124206