[VPlan] Add getIndexFor(Predecessor|Successor) helpers (NFC).

Move code to get the index of a predecessor and successor to helpers in
VPBlockBase, to avoid duplication and enable future reuse.

Split off from #140409.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -320,6 +320,20 @@ class VPBlockBase {
     std::swap(Successors[0], Successors[1]);
   }
 
+  /// Returns the index for \p Pred in the blocks predecessors list.
+  unsigned getIndexForPredecessor(const VPBlockBase *Pred) const {
+    assert(count(Predecessors, Pred) == 1 &&
+           "must have Pred exactly once in Predecessors");
+    return std::distance(Predecessors.begin(), find(Predecessors, Pred));
+  }
+
+  /// Returns the index for \p Succ in the blocks successor list.
+  unsigned getIndexForSuccessor(const VPBlockBase *Succ) const {
+    assert(count(Successors, Succ) == 1 &&
+           "must have Succ exactly once in Successors");
+    return std::distance(Successors.begin(), find(Successors, Succ));
+  }
+
   /// The method which generates the output IR that correspond to this
   /// VPBlockBase, thereby "executing" the VPlan.
   virtual void execute(VPTransformState *State) = 0;

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -1852,10 +1852,7 @@ static void removeBranchOnCondTrue(VPlan &Plan) {
       continue;
 
     VPBasicBlock *RemovedSucc = cast<VPBasicBlock>(VPBB->getSuccessors()[1]);
-    const auto &Preds = RemovedSucc->getPredecessors();
-    assert(count(Preds, VPBB) == 1 &&
-           "There must be a single edge between VPBB and its successor");
-    unsigned DeadIdx = std::distance(Preds.begin(), find(Preds, VPBB));
+    unsigned DeadIdx = RemovedSucc->getIndexForPredecessor(VPBB);
 
     // Values coming from VPBB into ResumePhi recipes of RemoveSucc are removed
     // from these recipes.

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -232,13 +232,8 @@ class VPBlockUtils {
   /// single edge between \p From and \p To.
   static void insertOnEdge(VPBlockBase *From, VPBlockBase *To,
                            VPBlockBase *BlockPtr) {
-    auto &Successors = From->getSuccessors();
-    auto &Predecessors = To->getPredecessors();
-    assert(count(Successors, To) == 1 && count(Predecessors, From) == 1 &&
-           "must have single between From and To");
-    unsigned SuccIdx = std::distance(Successors.begin(), find(Successors, To));
-    unsigned PredIx =
-        std::distance(Predecessors.begin(), find(Predecessors, From));
+    unsigned SuccIdx = From->getIndexForSuccessor(To);
+    unsigned PredIx = To->getIndexForPredecessor(From);
     VPBlockUtils::connectBlocks(From, BlockPtr, -1, SuccIdx);
     VPBlockUtils::connectBlocks(BlockPtr, To, PredIx, -1);
   }