Replace a few more "fall through" comments with LLVM_FALLTHROUGH

Follow up to r278902. I had missed "fall through", with a space.

llvm-svn: 278970
diff --git a/llvm/lib/Transforms/IPO/MergeFunctions.cpp b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
index fe653a7..726786a 100644
--- a/llvm/lib/Transforms/IPO/MergeFunctions.cpp
+++ b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
@@ -828,6 +828,7 @@
   default:
     llvm_unreachable("Unknown type!");
     // Fall through in Release mode.
+    LLVM_FALLTHROUGH;
   case Type::IntegerTyID:
     return cmpNumbers(cast<IntegerType>(TyL)->getBitWidth(),
                       cast<IntegerType>(TyR)->getBitWidth());