These are separate, mostly unrelated passes. Putting them in their own
files makes it easier to read the code, understand how to control the
passes, and makes it possible to independently trace, and debug them.
We need to walk the dominator tree starting at the header and update the
dominator of all nodes outside the loop we hit i.e the dominator tree nodes that
are immediately dominated 'by the loop' instead of only updating dominated exit
blocks.
rdar://34523864
