[ModuloSchedule] Introduce PeelingModuloScheduleExpander
This is the beginnings of a reimplementation of ModuloScheduleExpander. It works
by generating a single-block correct pipelined kernel and then peeling out the
prolog and epilogs.
This patch implements kernel generation as well as a validator that will
confirm the number of phis added is the same as the ModuloScheduleExpander.
Prolog and epilog peeling will come in a different patch.
Differential Revision: https://reviews.llvm.org/D67081