nir: add shader_info::io_lowered

This will drive decisions in many NIR passes and st/mesa.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6442>

src/compiler/shader_info.h

@@ -193,6 +193,11 @@ typedef struct shader_info {
    /* Whether flrp has been lowered. */
    bool flrp_lowered:1;
 
+   /* Whether nir_lower_io has been called to lower derefs.
+    * nir_variables for inputs and outputs might not be present in the IR.
+    */
+   bool io_lowered:1;
+
    /* Whether the shader writes memory, including transform feedback. */
    bool writes_memory:1;