Tighten the invariants around LoopBase::invalidate

Summary:
With this change:
 - Methods in LoopBase trip an assert if the receiver has been invalidated
 - LoopBase::clear frees up the memory held the LoopBase instance

This change also shuffles things around as necessary to work with this stricter invariant.

Reviewers: chandlerc

Subscribers: mehdi_amini, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D38055

llvm-svn: 313708