DateField cache should check the ExternalOperand instead of ExternalReference on X64

Review URL: https://chromiumcodereview.appspot.com/12637025
Patch from Weiliang Lin <weiliang.lin2@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13919 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Prepare push to trunk.  Now working on version 3.17.11.

R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/12827002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13920 ce2b1a6d-e550-0410-aec6-3dcde31c8c00