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

[via llvm-commits]

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@@ -3343,8 +3326,6 @@ void InnerLoopVectorizer::fixVectorizedLoop(VPTransformState &State,
                    VectorLoop->getHeader(), Plan, State);
   }
 
-  // Fix LCSSA phis not already fixed earlier. Extracts may need to be generated
-  // in the exit block, so update the builder.
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ayalz wrote:

Good to retain the first sentence: `// Fix live-out phis not already fixed earlier.`, (LCSSA holds only for exit block phis, now live-outs are fixed also for non LCSSA header phis in scalar header) and remove the following line
```
  State.Builder.SetInsertPoint(State.CFG.ExitBB,
                               State.CFG.ExitBB->getFirstNonPHIIt());
```
?
(also good to remove the subsequent loop sinking scalar operands, when ready :-)

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