[llvm] [VPlan] Remove ExtractLastLane for plans with scalar VFs. (PR #171145)

[via llvm-commits]

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@@ -383,7 +382,10 @@ void UnrollState::unrollBlock(VPBlockBase *VPB) {
         I->replaceAllUsesWith(getValueForPart(Op0, PartIdx));
         continue;
       }
-      // For vector VF, always extract from the last part.
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ayalz wrote:

Worth retaining the comment, perhaps emphasizing that
```// For vector VF, the penultimate element is always extracted from the last part.```

https://github.com/llvm/llvm-project/pull/171145