Removes all the direct uses of op->code so that we can change
the representation of "optab".  The code_to_optab change is
simply for uniformity (and better type checking).

        * optabs.h (struct optab_d): Rename code member to code_.
        (struct convert_optab_h): Likewise.
        (code_to_optab_): Rename from code_to_optab.
        (code_to_optab, optab_to_code): New.
        * dojump.c (do_compare_rtx_and_jump): Use code_to_optab.
        * ifcvt.c (noce_emit_move_insn): Likewise.
        * optabs.c (simplify_expand_binop): Use optab_to_code.
        (expand_simple_binop, shift_optab_p, commutative_optab_p): Likewise.
        (avoid_expensive_constant, expand_binop_directly): Likewise.
        (expand_binop, expand_simple_unop, expand_unop_direct): Likewise.
        (expand_unop, prepare_float_lib_cmp, expand_fixed_convert): Likewise.
        (have_insn_for, debug_optab_libfuncs): Likewise.
        (init_optab, init_optabv, init_convert_optab): Update for optab
        member name change.

From-SVN: r189818