Also partition side effects into observable and not observable, with only observable requiring Simulates and non-observable changes able to participate in GVN and code hoisting.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/8380017

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9847 ce2b1a6d-e550-0410-aec6-3dcde31c8c00