Use Succ->getEntryBasicBlock()->phis(), add comments

Author: lukel97

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -173,10 +173,7 @@ class VPBlockUtils {
       Succ->replacePredecessor(Old, New);
 
       // Replace any references to Old in widened phi incoming blocks.
-      while (auto *Region = dyn_cast<VPRegionBlock>(Succ))
-        Succ = Region->getEntry();
-
-      for (auto &R : *cast<VPBasicBlock>(Succ))
+      for (auto &R : Succ->getEntryBasicBlock()->phis())
         if (auto *WidenPhiR = dyn_cast<VPWidenPHIRecipe>(&R))
           for (unsigned I = 0; I < WidenPhiR->getNumOperands(); I++)
             if (WidenPhiR->getIncomingBlock(I) == Old)

llvm/unittests/Transforms/Vectorize/VPlanTest.cpp

@@ -661,6 +661,8 @@ TEST_F(VPBasicBlockTest, TraversingIteratorTest) {
 
 TEST_F(VPBasicBlockTest, reassociateBlocks) {
   {
+    // Ensure that when we reassociate a basic block, we make sure to update any
+    // references to it in VPWidenPHIRecipes' incoming blocks.
     VPlan &Plan = getPlan();
     VPBasicBlock *VPBB1 = Plan.createVPBasicBlock("VPBB1");
     VPBasicBlock *VPBB2 = Plan.createVPBasicBlock("VPBB2");
@@ -679,6 +681,8 @@ TEST_F(VPBasicBlockTest, reassociateBlocks) {
   }
 
   {
+    // Ensure that we update VPWidenPHIRecipes that are nested inside a
+    // VPRegionBlock.
     VPlan &Plan = getPlan();
     VPBasicBlock *VPBB1 = Plan.createVPBasicBlock("VPBB1");
     VPBasicBlock *VPBB2 = Plan.createVPBasicBlock("VPBB2");