Fix "existant" typos

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193579 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index f7db618..176e850 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -6451,7 +6451,7 @@
 
 void llvm::checkForCycles(const llvm::SDNode *N) {
 #ifdef XDEBUG
-  assert(N && "Checking nonexistant SDNode");
+  assert(N && "Checking nonexistent SDNode");
   SmallPtrSet<const SDNode*, 32> visited;
   SmallPtrSet<const SDNode*, 32> checked;
   checkForCyclesHelper(N, visited, checked);