Rename usesCustomDAGSchedInserter to usesCustomInserter, and update a

Rename usesCustomDAGSchedInserter to usesCustomInserter, and update a
bunch of associated comments, because it doesn't have anything to do
with DAGs or scheduling. This is another step in decoupling MachineInstr
emitting from scheduling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85517 91177308-0d34-0410-b5e6-96231b3b80d8