[llvm] [VPlan] Thread plan to VPBuilder (NFC) (PR #125364)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> > We should always have a valid instruction point when we create a recipe I think. Can we get the plan when needed on demand?
> 
> Makes sense, will check why my previous attempt was crashing again.

The builder seems to be used in two ways: to create a new recipe, and to create a new recipe and insert it:

```cpp
  template <typename T> T *tryInsertInstruction(T *R) {
    if (BB)
      BB->insert(R, InsertPt);
    return R;
  }
```

... but I agree that we need to fix this issue instead of doing the NFC of passing the Plan to VPBuilder.

https://github.com/llvm/llvm-project/pull/125364