[llvm] [VPlan][NFC] Speed up getVectorLoopRegion() with a last-successor walk (PR #199437)

[Vladimir Vereschaka via llvm-commits]

vvereschaka wrote:

Hi @madhur13490 , one more expensive check builder with the failed test
https://lab.llvm.org/buildbot/#/builders/187/builds/22485
```
******************** TEST 'LLVM-Unit :: Transforms/Vectorize/./VectorizeTests/57/66' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/buildbot/worker/expensive-checks/build/unittests/Transforms/Vectorize/./VectorizeTests-LLVM-Unit-376237-57-66.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=66 GTEST_SHARD_INDEX=57 /home/buildbot/worker/expensive-checks/build/unittests/Transforms/Vectorize/./VectorizeTests
--
Note: This is test shard 58 of 66.
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from VPVerifierTest
[ RUN      ] VPVerifierTest.DuplicateSuccessorsOutsideRegion
VectorizeTests: /home/buildbot/worker/expensive-checks/llvm-project/llvm/lib/Transforms/Vectorize/VPlan.cpp:1107: VPRegionBlock *llvm::VPlan::getVectorLoopRegion(): Assertion `LoopRegion == findVectorLoopRegionByScan(getEntry()) && "fast vector loop region lookup disagrees with full CFG scan"' failed.
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  VectorizeTests 0x00005f336588091b
1  VectorizeTests 0x00005f336587dc35
2  VectorizeTests 0x00005f336588177b
3  libc.so.6      0x00007b5f0259c330
4  libc.so.6      0x00007b5f025f5b2c pthread_kill + 284
5  libc.so.6      0x00007b5f0259c27e gsignal + 30
6  libc.so.6      0x00007b5f0257f8ff abort + 223
7  libc.so.6      0x00007b5f0257f81b
8  libc.so.6      0x00007b5f02592517
9  VectorizeTests 0x00005f336571958c
10 VectorizeTests 0x00005f3365764283
11 VectorizeTests 0x00005f336544c5a9
12 VectorizeTests 0x00005f3365895d60
13 VectorizeTests 0x00005f3365896b50
14 VectorizeTests 0x00005f3365897763
15 VectorizeTests 0x00005f33658a8984
16 VectorizeTests 0x00005f33658a7d49
17 VectorizeTests 0x00005f336588212a
18 libc.so.6      0x00007b5f025811ca
19 libc.so.6      0x00007b5f0258128b __libc_start_main + 139
20 VectorizeTests 0x00005f33653cf8d5
--
exit: -6
--
shard JSON output does not exist: /home/buildbot/worker/expensive-checks/build/unittests/Transforms/Vectorize/./VectorizeTests-LLVM-Unit-376237-57-66.json
********************
```

https://github.com/llvm/llvm-project/pull/199437