ScheduleDAGInstrs: Rework schedule graph builder.

ScheduleDAGInstrs: Rework schedule graph builder.

Re-comitting with a change that avoids undefined uses getting put into
the VRegUses list.

The new algorithm remembers the uses encountered while walking backwards
until a matching def is found. Contrary to the previous version this:
- Works without LiveIntervals being available
- Allows to increase the precision to subregisters/lanemasks
  (not used for now)

The changes in the AMDGPU tests are necessary because the R600 scheduler
is not stable with respect to the order of nodes in the ready queues.

Differential Revision: http://reviews.llvm.org/D9068

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254683 91177308-0d34-0410-b5e6-96231b3b80d8