[llvm] [VPlan] Build initial VPlan 0 using HCFGBuilder for inner loops. (NFC) (PR #124432)

[via llvm-commits]

Zentrik wrote:

I'm getting an assertion failure after this commit when using llvm on Linux and Windows. I can create a reproducer if that's useful. 

```
Assertion failed: i < getNumSuccessors() && "Successor # out of range for Branch!", file /workspace/srcdir/llvm-project/llvm/include/llvm/IR/Instructions.h, line 3117

[5772] signal 22: SIGABRT
in expression starting at none:0
crt_sig_handler at D:/a/llvm_julia_tester/llvm_julia_tester/julia/src\signals-win.c:104
raise at C:\Windows\System32\msvcrt.dll (unknown line)
abort at C:\Windows\System32\msvcrt.dll (unknown line)
assert at C:\Windows\System32\msvcrt.dll (unknown line)
_ZN12_GLOBAL__N_115PlainCFGBuilder13buildPlainCFGERN4llvm8DenseMapIPNS1_11VPBlockBaseEPNS1_10BasicBlockENS1_12DenseMapInfoIS4_vEENS1_6detail12DenseMapPairIS4_S6_EEEE at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZN4llvm16VPlanHCFGBuilder13buildPlainCFGEv at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZN4llvm16VPlanHCFGBuilder20buildHierarchicalCFGEv at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZN4llvm24LoopVectorizationPlanner24buildVPlansWithVPRecipesENS_12ElementCountES1_ at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZN4llvm24LoopVectorizationPlanner4planENS_12ElementCountEj at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZNK4llvm8LoopBaseINS_10BasicBlockENS_4LoopEE14getLoopLatchesERNS_15SmallVectorImplIPS1_EE at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZN4llvm28getOptionalBoolLoopAttributeEPKNS_4LoopENS_9StringRefE at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZN4llvm17LoopVectorizePass7runImplERNS_8FunctionE at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
_ZN4llvm17LoopVectorizePass3runERNS_8FunctionERNS_15AnalysisManagerIS1_JEEE at D:\a\llvm_julia_tester\llvm_julia_tester\julia\usr\bin\libLLVM-21jl.dll (unknown line)
```

https://github.com/llvm/llvm-project/pull/124432