[DAG] Don't increase SDNodeOrder for dbg.value/declare.

Summary:
The SDNodeOrder is saved in the IROrder field in the SDNode, and this
field may affects scheduling. Thus, letting dbg.value/declare increase
the order numbers may in turn affect scheduling.

Because of this change we also need to update the code deciding when
dbg values should be output, in ScheduleDAGSDNodes.cpp/ProcessSDDbgValues.

Dbg values now have the same order as the SDNode they are connected to,
not the following orders.

Test cases provided by Florian Hahn.

Reviewers: bogner, aprantl, sunfish, atrick

Reviewed By: atrick

Subscribers: fhahn, probinson, andreadb, llvm-commits, MatzeB

Differential Revision: https://reviews.llvm.org/D25318

llvm-svn: 292485