[llvm] [LV] Rework legacy widen/replicate in tryToBuildVPlan (NFC) (PR #224569)

[Vedant Paranjape via llvm-commits]

================
@@ -6610,50 +6604,44 @@ VPlanPtr LoopVectorizationPlanner::tryToBuildVPlan(VPlanPtr Plan,
   RUN_VPLAN_PASS(VPlanTransforms::makeCallWideningDecisions, *Plan, Range,
                  RecipeBuilder, CostCtx);
 
-  // Now process all other blocks and instructions.
-  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(RPOT)) {
-    // Convert input VPInstructions to widened recipes.
-    for (VPRecipeBase &R : make_early_inc_range(
-             make_range(VPBB->getFirstNonPhi(), VPBB->end()))) {
-      // Skip recipes that do not need transforming or have already been
-      // transformed.
-      if (isa<VPWidenCanonicalIVRecipe, VPBlendRecipe, VPReductionRecipe,
-              VPReplicateRecipe, VPWidenLoadRecipe, VPWidenStoreRecipe,
-              VPWidenCallRecipe, VPWidenIntrinsicRecipe, VPVectorPointerRecipe,
-              VPVectorEndPointerRecipe, VPHistogramRecipe>(&R) ||
-          (Instruction::isCast(cast<VPInstruction>(R).getOpcode()) &&
-           vputils::onlyFirstLaneUsed(R.getVPSingleValue())))
+  // Convert remaining VPInstructions to widen or replicate recipes.
+  // TODO: This legacy code should eventually be migrated to VPlan.
+  VPBasicBlock *HeaderVPBB = LoopRegion->getEntryBasicBlock();
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_shallow(HeaderVPBB))) {
+    for (VPInstruction &VPI :
+         make_early_inc_range(make_isa_range<VPInstruction>(*VPBB))) {
+      // We represent single-scalar casts directly as VPInstructions.
+      if (Instruction::isCast(VPI.getOpcode()) &&
+          vputils::onlyFirstLaneUsed(&VPI))
         continue;
-      auto *VPI = cast<VPInstruction>(&R);
-      if (!VPI->getUnderlyingValue())
+
+      Instruction *Instr = cast_or_null<Instruction>(VPI.getUnderlyingValue());
----------------
VedantParanjape wrote:

dyn_cast to TruncInst here and just check `!Instr` on L6629?

https://github.com/llvm/llvm-project/pull/224569