[globalisel] Sort RuleMatchers by priority.

Summary:
This makes more important rules have priority over less important rules.
For example, '%a = G_ADD $b:s64, $c:s64' has priority over
'%a = G_ADD $b:s32, $c:s32'. Previously these rules were emitted in the
correct order by chance.

NFC in this patch but it is required to make the next patch work correctly.

Depends on D29710

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

Reviewed By: ab, rovka

Subscribers: javed.absar, dberris, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D29711

llvm-svn: 296121