Label IDVS variants as being MESA_SHADER_{POSITION, VARYING} stages;
reserve the MESA_SHADER_VERTEX label for non-IDVS shaders. This reduces
confusion where a single shader compiles to two MESA_SHADER_VERTEX
shaders with different stats.
While we're at it, de-vendor the blend shader stage name; these stats
are internal anyway.
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15086>
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