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

[David Sherwood via llvm-commits]

================
@@ -7279,6 +7279,30 @@ static void fixReductionScalarResumeWhenVectorizingEpilog(
       BypassBlock, MainResumePhi->getIncomingValueForBlock(BypassBlock));
 }
 
+/// Add branch weight metadata, if the \p Plan's middle block is terminated by a
+/// BranchOnCond recipe.
+static void addBranchWeigthToMiddleTerminator(VPlan &Plan, ElementCount VF,
+                                              Loop *OrigLoop) {
+  // 4. Adjust branch weight of the branch in the middle block.
+  Instruction *LatchTerm = OrigLoop->getLoopLatch()->getTerminator();
+  if (!hasBranchWeightMD(*LatchTerm))
+    return;
+
+  VPBasicBlock *MiddleVPBB = Plan.getMiddleBlock();
+  auto *MiddleTerm =
+      dyn_cast_or_null<VPInstruction>(MiddleVPBB->getTerminator());
----------------
david-arm wrote:

The code that was removed look like this:

```
    auto *MiddleTerm =
        cast<BranchInst>(State.CFG.VPBB2IRBB[MiddleVPBB]->getTerminator());
```

whereas the code you've added here suggests that there may not be a terminator at all, or it may not be a VPInstruction. It may be right, but it's not obvious to me at least how this is NFC? I'm a bit worried that we may not be adding branch weights any more for some loops.

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