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.

llvm-svn: 231980