[llvm] [NFC][CodeGen] Use DenseMap instead of unordered_map in BasicBlockMatchingAndInference (PR #192824)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `arc-builder` running on `arc-worker` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/3/builds/31237

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

```
Step 6 (test-build-unified-tree-check-all) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
1.145 [48/18/6] Linking CXX executable unittests/ExecutionEngine/ExecutionEngineTests
1.315 [47/18/7] Linking CXX executable unittests/DebugInfo/BTF/DebugInfoBTFTests
1.433 [46/18/8] Linking CXX executable unittests/DebugInfo/PDB/DebugInfoPDBTests
1.830 [45/18/9] Linking CXX executable tools/clang/unittests/Format/FormatTests
3.674 [44/18/10] cd /buildbot/worker/arc-folder/llvm-project/clang/bindings/python && /usr/local/bin/cmake -E env CLANG_NO_DEFAULT_CONFIG=1 LIBCLANG_LIBRARY_PATH=/buildbot/worker/arc-folder/build/lib /usr/local/bin/python3.9 -m unittest discover
.....................................................................................................................................................................................
----------------------------------------------------------------------
Ran 181 tests in 3.459s

OK
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1422.320658

```

</details>

https://github.com/llvm/llvm-project/pull/192824