Re-introduce this as a trivial expression

The reverts r3939 as it turned out not be the root cause for the regression http://crbug.com/36604.
Review URL: http://codereview.chromium.org/661365

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3993 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ast.h

@@ -931,6 +931,10 @@ class VariableProxy: public Expression {
     return var()->is_global() || var()->rewrite()->IsLeaf();
   }
 
+  // Reading from a mutable variable is a side effect, but 'this' is
+  // immutable.
+  virtual bool IsTrivial() { return is_this(); }
+
   bool IsVariable(Handle<String> n) {
     return !is_this() && name().is_identical_to(n);
   }