[PM] Teach the LoopPassManager to automatically canonicalize loops by

runnig LCSSA over them prior to running the loop pipeline.

This also teaches the loop PM to verify that LCSSA form is preserved
throughout the pipeline's run across the loop nest.

Most of the test updates just leverage this new functionality. One has to be
relaxed with the new PM as IVUsers is less powerful when it sees LCSSA input.

Differential Revision: https://reviews.llvm.org/D28743

llvm-svn: 292241