[llvm] [VPlan] Consistently use (Part, 0) for first lane scalar values (PR #80271)

[via llvm-commits]

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@@ -213,8 +213,13 @@ VPBasicBlock::iterator VPBasicBlock::getFirstNonPhi() {
 }
 
 Value *VPTransformState::get(VPValue *Def, const VPIteration &Instance) {
-  if (Def->isLiveIn())
-    return Def->getLiveInIRValue();
+  if (Def->isLiveIn()) {
+    if (Value *V = Def->getLiveInIRValue())
+      return V;
+    if (hasScalarValue(Def, VPIteration(0, 0))) {
+      return Data.PerPartScalars[Def][0][0];
+    }
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ayalz wrote:

else?
should this if be an assert?

https://github.com/llvm/llvm-project/pull/80271