[LoopInterchange] Preserve LCSSA.

This patch extends LoopInterchange to move LCSSA to the right place
after interchanging. This is required for LoopInterchange to become a
function pass.

An alternative to the manual moving of the PHIs, we could also re-form
the LCSSA phis for a set of interchanged loops, but that's more
expensive.

Reviewers: efriedma, mcrosier, davide

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D52154

llvm-svn: 343132