Refactored ComputeLoadStoreField a bit on the way to clarify a bit what it
actually does.

Review URL: https://chromiumcodereview.appspot.com/10692187

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12072 ce2b1a6d-e550-0410-aec6-3dcde31c8c00