[GlobalISel][Legalizer] NFC mostly reducing LegalizeRuleSet's methods' inter-dependecies

Making LegalizeRuleSet's implementation a little more dumb and
straightforward to make it easier to read and change, in particular in
order to add the initial version of LegalizerInfo verifier

Reviewers: aemerson, qcolombet

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D46338

llvm-svn: 333562