Change the scheduler accessor methods to accept an explicit TargetMachine

Change the scheduler accessor methods to accept an explicit TargetMachine
argument instead of taking the SelectionDAG's TargetMachine. This is
needed for some upcoming scheduler changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59055 91177308-0d34-0410-b5e6-96231b3b80d8