Added LLVM_FALLTHROUGH to address warning: this statement may fall through. NFC.

llvm-svn: 304637

lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -1823,6 +1823,7 @@ static PHINode *getLoopPhiForCounter(Value *IncV, Loop *L, DominatorTree *DT) {
     // An IV counter must preserve its type.
     if (IncI->getNumOperands() == 2)
       break;
+    LLVM_FALLTHROUGH;
   default:
     return nullptr;
   }