Add lowering for loop.parallel to cfg.
This also removes the explicit pattern for loop.terminator to ensure
that the terminator is only erased if the parent op is rewritten.
Reductions are not yet supported.
Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits
Differential Revision: https://reviews.llvm.org/D73348