Make Loop and MachineLoop be subclasses of LoopBase, rather than typedefs,

Make Loop and MachineLoop be subclasses of LoopBase, rather than typedefs,
using the Curiously Recurring Template Pattern with LoopBase.

This will help further refactoring, and future functionality for
Loop. Also, Headers can now foward-declare Loop, instead of pulling
in LoopInfo.h or doing tricks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75519 91177308-0d34-0410-b5e6-96231b3b80d8