invoke.texi: Document -frename-registers.

invoke.texi: Document -frename-registers.

        * invoke.texi: Document -frename-registers.  Add it to -O3.
        * toplev.c (rest_of_compilation): Run regrename before ifcvt2.
        (enum dump_file_index, dump_file): Update order.
        (main): Set flag_rename_registers at -O3.

From-SVN: r37092