Add a readonly flag for loads (#562)

* Add readonly MemFlag

* Add readonly flag verifier check

* Make global loads readonly

* Fix gvn to consider readonly loads

lib/wasm/src/environ/dummy.rs

@@ -199,6 +199,7 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
             base: addr,
             offset: Offset32::new(0),
             global_type: self.pointer_type(),
+            readonly: true,
         });
 
         func.create_heap(ir::HeapData {
@@ -219,11 +220,13 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
             base: vmctx,
             offset: Offset32::new(0),
             global_type: self.pointer_type(),
+            readonly: true, // when tables in wasm become "growable", revisit whether this can be readonly or not.
         });
         let bound_gv = func.create_global_value(ir::GlobalValueData::Load {
             base: vmctx,
             offset: Offset32::new(0),
             global_type: I32,
+            readonly: true,
         });
 
         func.create_table(ir::TableData {