Introduce and use unknown_optab

Introduce and use unknown_optab

Prep for a later patch where this becomes an enum value.

        * optabs.h (unknown_optab): New.
        * builtins.c (interclass_mathfn_icode): Use it.
        * dojump.c (do_compare_rtx_and_jump): Likewise.
        * optabs.c (optab_for_tree_code, expand_binop): Likewise.
        * tree-vect-generic.c (add_rshift, expand_vector_divmod): Likewise.
        (expand_vector_operations_1): Likewise.
        * tree-vect-patterns.c (vect_recog_divmod_pattern): Likewise.
        * tree-vect-stmts.c (supportable_narrowing_operation): Likewise.

From-SVN: r189816