[AArch64] Fix unannotated fall-through between switch labels

This is breaking buildbot with -Werror,-Wimplicit-fallthrough on.
eg:
http://lab.llvm.org:8011/builders/ppc64le-lld-multistage-test/builds/6881

llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp

@@ -153,6 +153,7 @@ public:
       if (auto CN = dyn_cast<ConstantFPSDNode>(Opnd0))
         if (CN->isZero())
           return true;
+      break;
     }
     default:
       break;