[Loops] Restructure the LoopInfo verify function so that it more

directly walks the current loop structure verifying that a matching
structure can be found in a freshly computed version.

Also pull things out of containers when necessary once an issue is found
and print them directly.

This makes it substantially easier to debug verification failures as
the process stops at the exact point in the loop nest where they diverge
and has in easily accessed local variables (or printed to stderr
already) the loops and other information needed to analyze the failure.

Differential Revision: https://reviews.llvm.org/D29142

llvm-svn: 293133