[GlobalISel] Make LegalizerInfo accessible in LegalizerHelper

Summary:
We don’t actually use LegalizerInfo in Legalizer pass, it’s just passed
as an argument.

In order to check if an instruction is legal or not, we need to get LegalizerInfo
by calling `MI.getParent()->getParent()->getSubtarget().getLegalizerInfo()`.
Instead, make LegalizerInfo accessible in LegalizerHelper.

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

Reviewed By: qcolombet

Subscribers: dberris, llvm-commits, rovka

Differential Revision: https://reviews.llvm.org/D30838

llvm-svn: 297491