[globalisel][combine] Split existing rules into a match and apply step

Summary:
The declarative tablegen definitions split rules into match and apply steps.
Prepare for that by doing the same in the C++ implementations. This aids
some of the migration effort while the tablegen version is incomplete.

Reviewers: bogner, volkan, aditya_nandakumar, paquette, aemerson

Reviewed By: aditya_nandakumar

Subscribers: rovka, kristof.beyls, Petar.Avramovic, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58150

llvm-svn: 353996