We only want it to trigger if MRT is actually in use. This is a cheap
key (only require multiple variants for an obscure edge case) and avoids
the perf regression of using this pass which is needed for conformance.
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10393>