[LoopInterchange] Add more debug messages to currentLimitations().

Summary: This makes it easier to find out which limitation prevented this pass from doing its work.

Reviewers: karthikthecool, mzolotukhin, efriedma, mcrosier

Reviewed By: mcrosier

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D34940

llvm-svn: 307035