The old modifiesState method really checked if an operator was an assignment,
so restored that behaviour and use the new side effects detection only for
the new code.

ANGLEBUG=486
BUG=
R=nicolascapens@chromium.org, zmo@chromium.org

Review URL: https://codereview.appspot.com/22130043

Change-Id: I84d4e95a0457e63f237a814d80e4f72dd861496b