Quick: Avoid unnecessary GVN work in release builds.

In GVN's post-processing phase, compare LVNs only in debug
builds as they should be equal anyway.

Remove the Gate() from GVN cleanup pass and remove the
DCHECK() from MIRGraph::GlobalValueNumberingCleanup()
to make it a no-op if the GVN didn't run.

Bug: 16398693

(cherry picked from commit f725550c)

Change-Id: I518fba4a06c8d6d5ab16a6c122dc680b6d44814b