PM: Implement a basic loop pass manager

This creates the new-style LoopPassManager and wires it up with dummy
and print passes.

This version doesn't support modifying the loop nest at all. It will
be far easier to discuss and evaluate the approaches to that with this
in place so that the boilerplate is out of the way.

llvm-svn: 261831
GitOrigin-RevId: eecc3c826a58cb96697e56e159b14c7e9ae605dc
12 files changed