Add lowering for loop.parallel to cfg.

Authored by Stephan Herhut <herhut@google.com> on Jan 24 2020, 6:17 AM.


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.

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