nir: add a way to print the deref chain

Makes debugging easier when we care about the deref chain and not the
deref instruction itself.  To make it take a const pointer, constify
some of the static functions in nir_print.c.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/nir/nir.h

@@ -2725,6 +2725,7 @@ void nir_index_blocks(nir_function_impl *impl);
 void nir_print_shader(nir_shader *shader, FILE *fp);
 void nir_print_shader_annotated(nir_shader *shader, FILE *fp, struct hash_table *errors);
 void nir_print_instr(const nir_instr *instr, FILE *fp);
+void nir_print_deref(const nir_deref_instr *deref, FILE *fp);
 
 nir_shader *nir_shader_clone(void *mem_ctx, const nir_shader *s);
 nir_function_impl *nir_function_impl_clone(const nir_function_impl *fi);