[globalisel][tablegen] Add support for COPY_TO_REGCLASS.

Summary:
As part of this
* Emitted instructions now have named MachineInstr variables associated
  with them. This isn't particularly important yet but it's a small step
  towards multiple-insn emission.
* constrainSelectedInstRegOperands() is no longer hardcoded. It's now added
  as the ConstrainOperandsToDefinitionAction() action. COPY_TO_REGCLASS uses
  an alternate constraint mechanism ConstrainOperandToRegClassAction() which
  supports arbitrary constraints such as that defined by COPY_TO_REGCLASS.

Reviewers: ab, qcolombet, t.p.northover, rovka, kristof.beyls, aditya_nandakumar

Reviewed By: ab

Subscribers: javed.absar, igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D33590

llvm-svn: 305791