iris: Eliminate iris_uncompiled_shader::needs_edge_flag

Use the flag that was set by nir_lower_passthrough_edgeflags.  The
lowering passes will soon be moved to a finalize_nir hook, so there
won't be any choice.  Ideally we'd like to eliminate iris_fix_edge_flags
completely, and this is a first step.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12858>

src/gallium/drivers/iris/iris_context.h

@@ -403,8 +403,6 @@ struct iris_uncompiled_shader {
    /** Have any shader variants been compiled yet? */
    bool compiled_once;
 
-   bool needs_edge_flag;
-
    /* Whether shader uses atomic operations. */
    bool uses_atomic_load_store;