[LegacyPM] Make the 'addLoop' method accept a loop to add rather than

having it internally allocate the loop.

This is a much more flexible API and necessary in the new loop unswitch
to reasonably support both new and old PMs in common code. It also just
seems like a cleaner separation of concerns.

NFC, this should just be a pure refactoring.

Differential Revision: https://reviews.llvm.org/D33528

llvm-svn: 303834