Enables GVN for x86 and x86_64.

Rationale:
As decided after the MIPS change, this change unifies our
six code generators again a bit (we cannot move it into
the generic path, since arm likes to run the simplifier
first). Generally the GVN does some last minute cleanup
(such as finding CSE in the runtime tests generated
by dynamic BCE). I started a golem run to find impact.

Test: test-art-host test-art-target
Change-Id: Ib4098c5bae2269e71fee95cc31e3662d3aa47f6a