[llvm] [VPlan] Introduce ExitPhi VPInstruction, use to create phi for FOR. (PR #94760)

[via llvm-commits]

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@@ -8522,6 +8476,58 @@ static void addUsersInExitBlock(VPBasicBlock *HeaderVPBB, Loop *OrigLoop,
   }
 }
 
+/// Feed a resume value for every FOR from the vector loop to the scalar loop,
+/// if middle block branches to scalar preheader, by introducing ExtractFromEnd
+/// and ResumePhi recipes in each, respectively, and a VPLiveOut which uses the
+/// latter and corresponds to the scalar header.
+static void addLiveOutsForFirstOrderRecurrences(VPlan &Plan) {
+  VPRegionBlock *VectorRegion = Plan.getVectorLoopRegion();
+
+  // Start by finding out if middle block branches to scalar preheader.
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ayalz wrote:

```suggestion
  // Start by finding out if middle block branches to scalar preheader, which is not a VPIRBasicBlock, unlike Exit block - the other possible successor of middle block.
```


https://github.com/llvm/llvm-project/pull/94760