VM: Fix frame size computation for optimized DBC code

runtime/vm/flow_graph_compiler_dbc.cc

Index: runtime/vm/flow_graph_compiler_dbc.cc
diff --git a/runtime/vm/flow_graph_compiler_dbc.cc b/runtime/vm/flow_graph_compiler_dbc.cc
index 5a60cb7d9b5a86c7e4056462ccfb305fdfbc53f7..af756db0895dcdf660228ed3c401865d6fc9ce0c 100644
--- a/runtime/vm/flow_graph_compiler_dbc.cc
+++ b/runtime/vm/flow_graph_compiler_dbc.cc
@@ -237,9 +237,9 @@ void FlowGraphCompiler::GenerateAssertAssignable(TokenPosition token_pos,
   AddCurrentDescriptor(RawPcDescriptors::kOther, deopt_id, token_pos);
   if (is_optimizing()) {
     // Assert assignable keeps the instance on the stack as the result,
-    // all other arguments are popped. Since there may have been a GC during
-    // the runtime call, we have to reload the instance.
-    __ PopLocal(locs->out(0).reg());
+    // all other arguments are popped.
+    ASSERT(locs->out(0).reg() == locs->in(0).reg());
+    __ Drop1();
   }
 }