ScheduleDAGInstrs: Move VRegUses to ScheduleDAGMILive; NFCI

Push VRegUses/collectVRegUses() down the class hierarchy towards its
only user ScheduleDAGMILive.

NFCI: The initialization of the map happens at a later point but that
should not matter.

This is in preparation to allow DAG mutators to merge nodes, which
relies on this map getting computed later.

llvm-svn: 286654