Metal: skip out variable rewrite for array element references. ANGLE_elem_ref already returns a reference which is incompatible with the ANGLE_out and ANGLE_inout builtins. Skip the rewrite for this builtin, similarly to swizzle_ref for which it was already skipped. Add a small regression test which catches this bug. Fixed: angleproject:7939 Change-Id: Iff8ead697220466ceb063ea2f8eebdb7d9e5896d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4152866 Commit-Queue: Jamie Madill <jmadill@chromium.org> Auto-Submit: Kenneth Russell <kbr@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>