Zero: fast accessors should handle both getters and setters

JDK-8140650 rebuilt the Method::is_accessor to accept all getter/setter forms. There is a leftover from that work that affects Zero: it only accepts getters. See:
 http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/1d42c245cc87#l4.8

It is unclear what is the benefit of extending Zero further, but in any case, it is worth exploring. Severin had volunteered to clean this up:
 http://mail.openjdk.java.net/pipermail/zero-dev/2015-November/000551.html