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/Target/Hexagon/HexagonSplitDouble.cpp b/llvm/lib/Target/Hexagon/HexagonSplitDouble.cpp
index f832ff2..e4e259b 100644
--- a/llvm/lib/Target/Hexagon/HexagonSplitDouble.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonSplitDouble.cpp
@@ -337,6 +337,7 @@
       if (V == 0 || V == -1)
         return 10;
       // Fall through into A2_combinew.
+      LLVM_FALLTHROUGH;
     }
     case Hexagon::A2_combinew:
       return 2;