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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373782 91177308-0d34-0410-b5e6-96231b3b80d8