Trace JSOP_THIS and JSOP_THISPROP.

Rename getprop to get_prop, just because.
Extract COMPUTE_THIS to jsinterp.h as JS_COMPUTE_THIS for reuse in tracer.

js/src/jsbuiltins.cpp

@@ -154,6 +154,15 @@ jsdouble FASTCALL builtin_Math_sqrt(jsdouble d)
     return sqrt(d);
 }
 
+JSObject* FASTCALL builtin_get_this(JSContext* cx)
+{
+    JSObject *obj;
+    JS_COMPUTE_THIS(cx, cx->fp, obj);
+    return obj;
+  error:
+    return NULL; /* XXX error */
+}
+
 #define LO ARGSIZE_LO
 #define F  ARGSIZE_F
 #define Q  ARGSIZE_Q