asahi,agx: Rewrite varying linking

Instead of using driver_location magic and hoping things work, make the
linkage between vertex and fragment shaders explicit. Thanks to the
coefficient register mechanism reverse-engineered and documented earlier
in this series, this does not require any shader keys to support
separable shaders. It just requires that we regenerate the coefficient
register binding tables at draw time, based on the varying layouts
decided by the compiler independently for the VS and FS. This is more
robust in the face of separate shaders.

This also gets us glProvokingVertex() support without shader keys.

After that, we don't need any of the remapping prepasses. For fragment
shaders, any old mapping will do, so we can assign coefficient registers
as we go (based on what the program actually uses, not nir_variable
information that might be stale by this point). We do want to cache
coefficient registers, particularly for fragcoord.w which is used for
perspective interpolation everywhere.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17198>

src/asahi/compiler/agx_compiler.h

@@ -375,9 +375,6 @@ typedef struct {
    struct agx_shader_info *out;
    struct agx_shader_key *key;
 
-   /* Remapping table for varyings indexed by driver_location */
-   unsigned varyings[AGX_MAX_VARYINGS];
-
    /* Place to start pushing new values */
    unsigned push_base;