RTL: Merge rtx_equal_p and hash_rtx functions with their callback variants

RTL: Merge rtx_equal_p and hash_rtx functions with their callback variants

Use default argument when callback function is not required to merge
rtx_equal_p and hash_rtx functions with their callback variants.

gcc/ChangeLog:

	* cse.cc (hash_rtx_cb): Rename to hash_rtx.
	(hash_rtx): Remove.
	* early-remat.cc (remat_candidate_hasher::equal): Update
	to call rtx_equal_p with rtx_equal_p_callback_function argument.
	* rtl.cc (rtx_equal_p_cb): Rename to rtx_equal_p.
	(rtx_equal_p): Remove.
	* rtl.h (rtx_equal_p): Add rtx_equal_p_callback_function
	argument with NULL default value.
	(rtx_equal_p_cb): Remove function declaration.
	(hash_rtx_cb): Ditto.
	(hash_rtx): Add hash_rtx_callback_function argument
	with NULL default value.
	* sel-sched-ir.cc (free_nop_pool): Update function comment.
	(skip_unspecs_callback): Ditto.
	(vinsn_init): Update to call hash_rtx with
	hash_rtx_callback_function argument.
	(vinsn_equal_p): Ditto.