Revert [VPlan] Consolidate logic for narrowToSingleScalars (llvm#170720)

This reverts commit 7b3ec51, as a crash was reported:
https://llvm.godbolt.org/z/dK6ff5zvr -- this will give us time to
investigate a re-land.

Author: artagnon

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -757,6 +757,31 @@ static void legalizeAndOptimizeInductions(VPlan &Plan) {
     if (!PhiR)
       continue;
 
+    // Try to narrow wide and replicating recipes to uniform recipes, based on
+    // VPlan analysis.
+    // TODO: Apply to all recipes in the future, to replace legacy uniformity
+    // analysis.
+    auto Users = collectUsersRecursively(PhiR);
+    for (VPUser *U : reverse(Users)) {
+      auto *Def = dyn_cast<VPRecipeWithIRFlags>(U);
+      auto *RepR = dyn_cast<VPReplicateRecipe>(U);
+      // Skip recipes that shouldn't be narrowed.
+      if (!Def || !isa<VPReplicateRecipe, VPWidenRecipe>(Def) ||
+          Def->getNumUsers() == 0 || !Def->getUnderlyingValue() ||
+          (RepR && (RepR->isSingleScalar() || RepR->isPredicated())))
+        continue;
+
+      // Skip recipes that may have other lanes than their first used.
+      if (!vputils::isSingleScalar(Def) && !vputils::onlyFirstLaneUsed(Def))
+        continue;
+
+      auto *Clone = new VPReplicateRecipe(Def->getUnderlyingInstr(),
+                                          Def->operands(), /*IsUniform*/ true,
+                                          /*Mask*/ nullptr, /*Flags*/ *Def);
+      Clone->insertAfter(Def);
+      Def->replaceAllUsesWith(Clone);
+    }
+
     // Replace wide pointer inductions which have only their scalars used by
     // PtrAdd(IndStart, ScalarIVSteps (0, Step)).
     if (auto *PtrIV = dyn_cast<VPWidenPointerInductionRecipe>(&Phi)) {
@@ -1522,11 +1547,8 @@ static void narrowToSingleScalarRecipes(VPlan &Plan) {
         continue;
       }
 
-      // Skip recipes that aren't single scalars and don't just have their first
-      // lane used.
-      if (!vputils::isSingleScalar(RepOrWidenR) &&
-          (!vputils::onlyFirstLaneUsed(RepOrWidenR) ||
-           RepOrWidenR->getNumUsers() == 0))
+      // Skip recipes that aren't single scalars.
+      if (!vputils::isSingleScalar(RepOrWidenR))
         continue;
 
       // Skip recipes for which conversion to single-scalar does introduce