Fix a performance bug in the register contraints for polymorphic loads.
Fix a performance bug in the register contraints for polymorphic loads.

The context is only needed in the esi register if there is a call to
the generic stub.
Review URL: https://chromiumcodereview.appspot.com/9467001

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@10819 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh