[VPlan] Restructure code for BranchOnCond codegen. (NFCI)

Reoder code to exit early if the BranchOnCond isn't in an exiting block.
This delays retrieving the parent region, which may not be present.

Split off from #92651.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -442,20 +442,20 @@ Value *VPInstruction::generatePerPart(VPTransformState &State, unsigned Part) {
       return nullptr;
 
     Value *Cond = State.get(getOperand(0), VPIteration(Part, 0));
-    VPRegionBlock *ParentRegion = getParent()->getParent();
-    VPBasicBlock *Header = ParentRegion->getEntryBasicBlock();
-
     // Replace the temporary unreachable terminator with a new conditional
     // branch, hooking it up to backward destination for exiting blocks now and
     // to forward destination(s) later when they are created.
     BranchInst *CondBr =
         Builder.CreateCondBr(Cond, Builder.GetInsertBlock(), nullptr);
-
-    if (getParent()->isExiting())
-      CondBr->setSuccessor(1, State.CFG.VPBB2IRBB[Header]);
-
     CondBr->setSuccessor(0, nullptr);
     Builder.GetInsertBlock()->getTerminator()->eraseFromParent();
+
+    if (!getParent()->isExiting())
+      return CondBr;
+
+    VPRegionBlock *ParentRegion = getParent()->getParent();
+    VPBasicBlock *Header = ParentRegion->getEntryBasicBlock();
+    CondBr->setSuccessor(1, State.CFG.VPBB2IRBB[Header]);
     return CondBr;
   }
   case VPInstruction::BranchOnCount: {