Patch to fix pragma metadata for do-while loops

Summary:
Make sure that loop metadata only is put on the backedge
when expanding a do-while loop.
Previously we added the loop metadata also on the branch
in the pre-header. That could confuse optimization passes
and result in the loop metadata being associated with the
wrong loop.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38011

Committing on behalf of deepak2427 (Deepak Panickal)

Reviewers: #clang, ABataev, hfinkel, aaron.ballman, bjope

Reviewed By: bjope

Subscribers: bjope, rsmith, shenhan, zzheng, xbolva00, lebedev.ri, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D48721

llvm-svn: 336717