[GlobalISel][AArch64] LegalizerInfo verifier: Adding...

[GlobalISel][AArch64] LegalizerInfo verifier: Adding LegalizerInfo::verify(...) call w/o fixing bugs

This is to make it clear what kind of bugs the LegalizerInfo::verifier
is able to catch and test its output

Reviewers: aemerson, qcolombet

Reviewed By: aemerson

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

llvm-svn: 333597