Determine which loop exit blocks need a 'pre-exit' block inserted.
Recognize when this would be impossible.

llvm-svn: 108941

llvm-svn: 108845

This is a work in progress. So far we have some basic loop analysis to help
determine where it is useful to split a live range around a loop.

The actual loop splitting code from Splitter.cpp is also going to move in here.

llvm-svn: 108842