nir: fix unused function warning in src/compiler/nir/nir.c

This commit fixes the following warning:
../src/compiler/nir/nir.c:1827:1: warning: ‘dest_is_ssa’ defined but not used [-Wunused-function]
 1827 | dest_is_ssa(nir_dest *dest, void *_state)
      | ^~~~~~~~~~~

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.c

@@ -1823,9 +1823,10 @@ cursor_next_instr(nir_cursor cursor)
    unreachable("Inavlid cursor option");
 }
 
-static bool
+ASSERTED static bool
 dest_is_ssa(nir_dest *dest, void *_state)
 {
+   (void) _state;
    return dest->is_ssa;
 }