[LCSSA] Add expensive verification of LCSSA form for sub-loops.

This assertion makes sure all sub-loops are in LCSSA form before
bringing their parent in LCSSA form. This precondition was added to
formLCSSA in D56848.

Reviewers: davide, efriedma, mzolotukhin

Reviewed By: davide

Differential Revision: https://reviews.llvm.org/D56921

llvm-svn: 352958