[llvm] VPlan/Builder: refine return types (NFC) (PR #108858)

[David Sherwood via llvm-commits]

david-arm wrote:

That's why IRBuilder.h defines functions like this:

```
Value *CreateAnd(Value *LHS, Value *RHS, const Twine &Name = "")
```

https://github.com/llvm/llvm-project/pull/108858