Remove useMachineScheduler and replace it with subtarget options

Remove useMachineScheduler and replace it with subtarget options
that control, individually, all of the disparate things it was
controlling.

At the same time move a FIXME in the Hexagon port to a new
subtarget function that will enable a user of the machine
scheduler to avoid using the source scheduler for pre-RA-scheduling.
The FIXME would have this removed, but involves either testcase
changes or adding -pre-RA-sched=source to a few testcases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231980 91177308-0d34-0410-b5e6-96231b3b80d8