Port r9847 (fc7590).

Original commit message:

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=
TEST=

Review URL: http://codereview.chromium.org/8387044
Patch from Gergely Kis <gergely@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9866 ce2b1a6d-e550-0410-aec6-3dcde31c8c00