Optimizing: Remove redundant hash set copy in GVN

During the GVN analysis, a basic block inherits the set of movable
instructions from its dominator. If the block is the only successor
of the dominating block, there is no need for cloning of the parent
set (a very expensive operation).

Change-Id: I59e033b9e9e093984dc8e903e3a7be1cb3645cc2