[ModuloSchedule] Make PeelingModuloScheduleExpander inheritable.

Basically a NFC, but allows subclasses access to the entire PeelingModuloScheduleExpander
class. We are doing this to allow backends, particularly one that are not necessarily
upstreamed, to inherit from PeelingModuloScheduleExpander and access its basic structures.

Renames Info into LoopInfo for consistency in PeelingModuloScheduleExpander.

Differential Revision: https://reviews.llvm.org/D82673