[globalisel][tablegen] Add support for RegisterOperand.

Summary:
It functions just like RegisterClass except that the class is obtained
from a field.

Depends on D31761.

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

Reviewed By: ab

Subscribers: dberris, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D32229

llvm-svn: 301080