[VPlan] Fix subtle issue with first-lane-used check

Author: artagnon

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1218,7 +1218,8 @@ static void narrowToSingleScalarRecipes(VPlan &Plan) {
       // Skip recipes that aren't single scalars, that don't have users, and
       // that don't have only their scalar results used (this would introduce
       // extra broadcasts).
-      if (!vputils::isSingleScalar(RepOrWidenR) ||
+      if ((!vputils::isSingleScalar(RepOrWidenR) &&
+           !vputils::onlyFirstLaneUsed(RepOrWidenR)) ||
           RepOrWidenR->getNumUsers() == 0 ||
           any_of(RepOrWidenR->users(), [RepOrWidenR](VPUser *U) {
             return !U->usesScalars(RepOrWidenR);

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -40,9 +40,6 @@ const SCEV *getSCEVExprForVPValue(VPValue *V, ScalarEvolution &SE);
 /// Returns true if \p VPV is a single scalar, either because it produces the
 /// same value for all lanes or only has its first lane used.
 inline bool isSingleScalar(const VPValue *VPV) {
-  if (onlyFirstLaneUsed(VPV))
-    return true;
-
   auto PreservesUniformity = [](unsigned Opcode) -> bool {
     if (Instruction::isBinaryOp(Opcode) || Instruction::isCast(Opcode))
       return true;