[GlobalISel][Legalizer] LegalizerInfo verifier: check rules cover type indices

This commit adds a simple verifier that tracks type indices being
touched by legalization rules' builders.

Every target will now have an opportunity to call
LegalizerInfo::verify(...) at the end of its derived LegalizerInfo's
constructor and check there are no obvious mistakes like checking only
first type for an opcode that has more than one type index and therefore
implicitly declaring any type for the second (and higher) type index
legal.

The check is only ran in assert builds and should have very minor
performance impact in assert builds and none in release builds.

This commit does not add LegalizerInfo::verify(...) calls to
target-specific legalizers, look for separate commits for that.

This commit also doesn't make the verification errors fatal, only
produces an error message, look for a later commit that does.

Reviewers: aemerson, qcolombet

Reviewed By: aemerson

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

llvm-svn: 333576