[LV] Retrieve middle VPBB via scalar ph to fix epilogue resumephis (NFC)

If ScalarPH has predecessors, we may need to update its reduction resume
values. If there is a middle block, it must be the first predecessor.
Note that the first predecessor may not be the middle block, if the
middle block doesn't branch to the scalar preheader. In that case,
fixReductionScalarResumeWhenVectorizingEpilog will be a no-op.

In preparation for #106748.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7786,7 +7786,6 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
 
   BestVPlan.execute(&State);
 
-  auto *MiddleVPBB = BestVPlan.getMiddleBlock();
   // 2.5 When vectorizing the epilogue, fix reduction resume values from the
   // additional bypass block.
   if (VectorizingEpilogue) {
@@ -7801,10 +7800,20 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
         Phi.addIncoming(Phi.getIncomingValueForBlock(BypassBlock), Pred);
       }
     }
-
-    for (VPRecipeBase &R : *MiddleVPBB) {
-      fixReductionScalarResumeWhenVectorizingEpilog(
-          &R, State, State.CFG.VPBB2IRBB[MiddleVPBB], BypassBlock);
+    VPBasicBlock *ScalarPH = BestVPlan.getScalarPreheader();
+    ArrayRef<VPBlockBase *> ScalarPreds = ScalarPH->getPredecessors();
+    if (!ScalarPreds.empty()) {
+      // If ScalarPH has predecessors, we may need to update its reduction
+      // resume values. If there is a middle block, it must be the first
+      // predecessor. Note that the first predecessor may not be the middle
+      // block, if the middle block doesn't branch to the scalar preheader. In
+      // that case, fixReductionScalarResumeWhenVectorizingEpilog will be a
+      // no-op.
+      auto *MiddleVPBB = cast<VPBasicBlock>(ScalarPreds[0]);
+      for (VPRecipeBase &R : *MiddleVPBB) {
+        fixReductionScalarResumeWhenVectorizingEpilog(
+            &R, State, State.CFG.VPBB2IRBB[MiddleVPBB], BypassBlock);
+      }
     }
   }