Hide the pre-RA-sched= option.

This is a very confusing option for a feature that will go away.

-enable-misched is exposed instead to help triage issues with the new
scheduler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199133 91177308-0d34-0410-b5e6-96231b3b80d8