[llvm] [VPlan] Set branch weight metadata on middle term in VPlan (NFC) (PR #143035)

[Hans Wennborg via llvm-commits]

zmodem wrote:

We're hitting assertion failures after this change:

```
      llvm/lib/Transforms/Vectorize/VPlan.h:4021:
      llvm::VPBasicBlock* llvm::VPlan::getMiddleBlock():
      Assertion `LoopRegion && "cannot call the function after vector loop region has been removed"' failed.
```

See https://crbug.com/424377400#comment3 for a reproducer.

I'll revert while it gets fixed.

https://github.com/llvm/llvm-project/pull/143035