[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

llvm-svn: 370893