verify-uselistorder: Call verifyModule() and improve output

Call `verifyModule()` after parsing and after every transformation.
Also convert some `DEBUG(dbgs())` to `errs()` to increase visibility
into what's going on.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215951 91177308-0d34-0410-b5e6-96231b3b80d8