nir/glsl: gather bitmask of images used by program

In a similar fashion as commit f5c7df4dc9 does for textures.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4080>

src/compiler/shader_info.h

@@ -140,6 +140,9 @@ typedef struct shader_info {
    /** Bitfield of which textures are used by texelFetch() */
    uint32_t textures_used_by_txf;
 
+   /** Bitfield of which images are used */
+   uint32_t images_used;
+
    /* SPV_KHR_float_controls: execution mode for floating point ops */
    uint16_t float_controls_execution_mode;