[GlobalISel] LegalizerInfo: Enable legalization of non-power-of-2 types

Summary: Legalize only if the type is marked as Legal or Custom. If not, return Unsupported as LegalizerHelper is not able to handle non-power-of-2 types right now.

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

Reviewed By: kristof.beyls, ab

Subscribers: dberris, rovka, igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D31711

llvm-svn: 299929