Introduce a varying load count pass to get the maximum amount of varying
loads from a fragment shader (prior to optimization passes), in order to
only allocate as many Attribute Descriptors as required. This will
generally lead to smaller buffers in SRT0 for fragment shaders.
As the amount of ADs is now dynamic based on the shader, we need to
lower varying loads early for fragment shaders in v9+, as the amount of
ADs will determine the offset for dummy_sampler, required during
nir_lower_descriptors.
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32969>
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