Vulkan: Retain OpSource in SPIR-V Needed by AGI. It's a small instruction, and there's only one of it. This change makes the SPIR-V transformer retain this instruction even on release. Bug: b/177556123 Change-Id: I0a2ba8afe55018b08f25abd8a139b64b2a5f6f11 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2636691 Reviewed-by: Ian Elliott <ianelliott@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>