Region Domain: MRU Cache

[LinerSu]

This PR includes the following updates: (1) Add a new variable domain that is used to represent the base address of an allocated object symbolically. The Hasse diagram is:

                   Top
        /     /     |     \   
...  var1   var2   ...  varn  ...
        \      \    |     /     
                   Bot

(2) Enable MRU cache in region domain. The basic process is during load_ref / store_ref: If load/store a data for a object by reference within a region:

• Check whether the object is an MRU object
  • If not, cache missed; commit cache to base domain; populate new MRU object into cache domain
• Perform strong read / update