[globalisel] Enable tracing the legalizer with --debug-only=legalize-mir

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

Reviewed By: kristof.beyls

Subscribers: dberris, igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D31750

llvm-svn: 300847