[llvm] [VPlan] Model FOR extract of exit value in VPlan. (PR #93395)

[via llvm-commits]

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@@ -558,6 +560,29 @@ Value *VPInstruction::generatePerPart(VPTransformState &State, unsigned Part) {
 
     return ReducedPartRdx;
   }
+  case VPInstruction::ExtractFromEnd: {
+    if (Part != 0)
+      return State.get(this, 0, /*IsScalar*/ true);
+
+    auto *CI = cast<ConstantInt>(getOperand(1)->getLiveInIRValue());
+    unsigned Offset = CI->getZExtValue();
+
+    // Extract lane VF - Offset in the from the operand.
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ayalz wrote:

```suggestion
    // Extract lane VF - Offset from the operand.
```
Better place under the "if", as it applies to the horizontal "then" case.

https://github.com/llvm/llvm-project/pull/93395