[Coroutines] Simplify implementation using removePredecessor

[junparser]

Summary

As the title says, this is just a minor update using existed api to simplify the code

[modocache]

LGTM, thanks!

llvm/lib/Transforms/Coroutines/CoroSplit.cpp

static bool simplifyTerminatorLeadingToRet(Instruction *InitialInst) {
   Instruction *I = InitialInst;
   while (I->isTerminator() ||
          (isa<CmpInst>(I) && I->getNextNode()->isTerminator())) {
     if (isa<ReturnInst>(I)) {
       if (I != InitialInst) {
         // If InitialInst is an unconditional branch,
         // remove PHI values that come from basic block of InitialInst
         if (UnconditionalSucc)
-          for (PHINode &PN : UnconditionalSucc->phis()) {
-            int idx = PN.getBasicBlockIndex(InitialInst->getParent());
-            if (idx != -1)
-              PN.removeIncomingValue(idx);
-          }
+          UnconditionalSucc->removePredecessor(InitialInst->getParent(), true);
         ReplaceInstWithInst(InitialInst, I->clone());
       }
       return true;
     }
     if (auto *BR = dyn_cast<BranchInst>(I)) {
       if (BR->isUnconditional()) {
         BasicBlock *BB = BR->getSuccessor(0);
         if (I == InitialInst)