CGLoopInfo: Use the MD_loop metadata kind from r264371, NFC

Besides a small compile-time speedup, there should be no real
functionality change here.

llvm-svn: 264372
diff --git a/clang/lib/CodeGen/CGLoopInfo.cpp b/clang/lib/CodeGen/CGLoopInfo.cpp
index f10cfcf..08ed173 100644
--- a/clang/lib/CodeGen/CGLoopInfo.cpp
+++ b/clang/lib/CodeGen/CGLoopInfo.cpp
@@ -258,7 +258,7 @@
   if (TerminatorInst *TI = dyn_cast<TerminatorInst>(I)) {
     for (unsigned i = 0, ie = TI->getNumSuccessors(); i < ie; ++i)
       if (TI->getSuccessor(i) == L.getHeader()) {
-        TI->setMetadata("llvm.loop", L.getLoopID());
+        TI->setMetadata(llvm::LLVMContext::MD_loop, L.getLoopID());
         break;
       }
     return;