[ScheduleDAG] When a node is cloned, add an edge between the nodes.

InstrEmitter's virtual register handling assumes that clones are emitted
after the cloned node.  Make sure this assumption actually holds.

Fixes a "Node emitted out of order - early" assertion on the testcase.

This is probably a very rare case to actually hit in practice; even
without the explicit edge, the scheduler will usually end up scheduling
the nodes in the expected order due to other constraints.

Differential Revision: https://reviews.llvm.org/D68068

llvm-svn: 373782