* rtl.def (CONSTANT_P_RTX): New.
        * rtl.h (CONSTANT_P): Recognize it.
        * cse.c (fold_rtx): Eliminate it.
        * expr.c (can_handle_constant_p): New variable.
        (init_expr_once): Initialize it.
        (expand_builtin): Generate CONSTANT_P_RTX if the expression is not
        immediately recognizable as a constant.

From-SVN: r20846