Add lowering for loop.parallel to cfg.

Summary:
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.

Reviewers: nicolasvasilache

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73348