[llvm] [VPlan] Migrate VPBuilder to VPInsertPt fully (NFC) (PR #209764)

[Luke Lau via llvm-commits]

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@@ -97,13 +97,52 @@ void reportVectorization(OptimizationRemarkEmitter *ORE, Loop *TheLoop,
 
 /// VPlan-based builder utility analogous to IRBuilder.
 class VPBuilder {
-  VPBasicBlock *BB = nullptr;
-  VPBasicBlock::iterator InsertPt = VPBasicBlock::iterator();
+public:
+  /// InsertPoint - A saved insertion point.
+  class VPInsertPoint {
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lukel97 wrote:

I'm not sure I understand the motivation behind this. It doesn't simplify any of the call sites and it ends up exposing what was previously an implementation details.

IRBuilder just uses BasicBlocks and BasicBlock::iterators in the constructor:

```c++
  IRBuilder(BasicBlock *TheBB, BasicBlock::iterator IP, FolderTy Folder,
            MDNode *FPMathTag = nullptr,
            ArrayRef<OperandBundleDef> OpBundles = {})
```

https://github.com/llvm/llvm-project/pull/209764