Add SchedRW as an Instruction field.

Don't require instructions to inherit Sched<...>. Sometimes it is more
convenient to say:

  let SchedRW = ... in {
    ...
  }

Which is now possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177199 91177308-0d34-0410-b5e6-96231b3b80d8