D3D: Make non-constant loop indexing limitation a feature flag ... instead of exposing it as a front-end limitation only to set a shader flag in the front-end. Bug: angleproject:1130 Change-Id: Idbd9a1b7b79d231bd9fc1c0061c768df4aaab92d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5004847 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>