nir: Don't set writes_memory for reading XFB

nir: Don't set writes_memory for reading XFB

That's a read, not a write. Fixes optimizations getting disabled for fragment
shaders when linked with a shader producing transform feedback varyings.

Fixes: 85a723975bb ("nir: add and gather shader_info::writes_memory")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16285>