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

[via llvm-commits]

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@@ -167,8 +167,8 @@ 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) {
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ayalz wrote:

This works "horizontally" for Offsets between 1 and VF, inclusive, if VF >= 2,
and "vertically" for Offsets between 1 and UF, inclusive, if VF = 1, implying UF >= 2.
I.e., this works in general only for Offset = 1 and Offset = 2 across possible {VF, UF} combinations.
May be good to keep private and expose only public `last` and `penultimate` cases. And/or assert.


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