iris: Rework edgeflag handling

We were relying on specific pass ordering in st to avoid setting
inputs_read/outputs_written for edge flags.  Instead, just assume
that it happens and throw out the results we don't want.

We should probably revisit this and try and add a vertex element
property like I originally wanted so we can avoid having it be
associated with the VS altogether.

src/gallium/drivers/iris/iris_context.h

@@ -281,6 +281,8 @@ struct iris_uncompiled_shader {
    /** Should we use ALT mode for math?  Useful for ARB programs. */
    bool use_alt_mode;
 
+   bool needs_edge_flag;
+
    /** Constant data scraped from the shader by nir_opt_large_constants */
    struct pipe_resource *const_data;