[ObjCARC] Convert an if to an early continue. NFC

[smeenai]

Summary

This reduces nesting and is consistent with a planned follow-up change.

[ahatanak]

LGTM

lib/Transforms/ObjCARC/ObjCARCContract.cpp

auto ReplaceArgUses = [Inst, this](Value *Arg) {
         unsigned OperandNo = U.getOperandNo();
 
         // If the call's return value dominates a use of the call's argument
         // value, rewrite the use to use the return value. We check for
         // reachability here because an unreachable call is considered to
         // trivially dominate itself, which would lead us to rewriting its
         // argument in terms of its return value, which would lead to
         // infinite loops in GetArgRCIdentityRoot.
-        if (DT->isReachableFromEntry(U) && DT->dominates(Inst, U)) {
-          Changed = true;
-          Instruction *Replacement = Inst;
-          Type *UseTy = U.get()->getType();
-          if (PHINode *PHI = dyn_cast<PHINode>(U.getUser())) {
-            // For PHI nodes, insert the bitcast in the predecessor block.
-            unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
-            BasicBlock *BB = PHI->getIncomingBlock(ValNo);
-            if (Replacement->getType() != UseTy)
-              Replacement = new BitCastInst(Replacement, UseTy, "",
-                                            &BB->back());
-            // While we're here, rewrite all edges for this PHI, rather
-            // than just one use at a time, to minimize the number of
-            // bitcasts we emit.
-            for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i)
-              if (PHI->getIncomingBlock(i) == BB) {
-                // Keep the UI iterator valid.
-                if (UI != UE &&
-                    &PHI->getOperandUse(
-                        PHINode::getOperandNumForIncomingValue(i)) == &*UI)
-                  ++UI;
-                PHI->setIncomingValue(i, Replacement);
-              }
-          } else {
-            if (Replacement->getType() != UseTy)
-              Replacement = new BitCastInst(Replacement, UseTy, "",
-                                            cast<Instruction>(U.getUser()));
-            U.set(Replacement);
-          }
-        }
-      }
+        if (!DT->isReachableFromEntry(U) || !DT->dominates(Inst, U))
+          continue;
+
+            Replacement = new BitCastInst(Replacement, UseTy, "", &BB->back());
     };
 
 
     Value *Arg = cast<CallInst>(Inst)->getArgOperand(0);
     Value *OrigArg = Arg;
 
     // TODO: Change this to a do-while.
     for (;;) {