re PR middle-end/17503 (quadratic behaviour in invalid_mode_change_p)

        PR rtl-opt/17503
        * regclass.c (subregs_of_mode): Turn into an htab.  Make static.
        (som_hash, som_eq): New.
        (init_subregs_of_mode, record_subregs_of_mode): New.
        (cannot_change_mode_set_regs): Rewrite for htab implementation.
        (invalid_mode_change_p): Likewise.
        * combine.c (gen_lowpart_for_combine): Use record_subregs_of_mode.
        * flow.c (mark_used_regs): Likewise.
        (life_analysis): Use init_subregs_of_mode.
        * regs.h (subregs_of_mode): Remove.
        * rtl.h (init_subregs_of_mode, record_subregs_of_mode): Declare.

From-SVN: r88067