[MachineScheduler] Dump SUnits before calling SchedImpl->initialize()

This is a NFC simple patch that changes the DEBUG dumping in the
MachineScheduler so that the dumping of the built SUnits is done before the
SchedImpl->initialize() is called.

This is better on SystemZ, since it has a strategy that does some dumping at
the start of the region, and it is not possible to easily read it if it is
output above a long list of SU.

Review: Javed Absar
https://reviews.llvm.org/D44089

llvm-svn: 326716