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

[via llvm-commits]

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@@ -167,8 +167,10 @@ class VPLane {
 
   static VPLane getFirstLane() { return VPLane(0, VPLane::Kind::First); }
 
-  static VPLane getLastLaneForVF(const ElementCount &VF) {
-    unsigned LaneOffset = VF.getKnownMinValue() - 1;
+  static VPLane getLaneFromEnd(const ElementCount &VF, unsigned Offset) {
+    assert(Offset <= VF.getKnownMinValue() &&
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ayalz wrote:

```suggestion
    assert(Offset > 0 && Offset <= VF.getKnownMinValue() &&
```

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