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.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

OK for master?

Uros.