[GCC ARM-Embedded-4_7]Back port r193425(improving reger pressure hoisting) from TRUNK

[Bin Cheng]

Hi,
I back ported r193425 from TRUNK to GCC ARM-Embedded-4_7 branch as r193495.

Thanks

        Backport from mainline r193425
        2012-11-12  Bin Cheng  <bin.ch...@arm.com>

        * gcse.c (struct bb_data): Add new fields, old_pressure, live_in
        and backup.
        (get_regno_pressure_class): Add prototype.
        (update_bb_reg_pressure): New.
        (should_hoist_expr_to_dom): Add new parameter from.
        Monitor the change of reg pressure and use it to drive hoisting.
        (hoist_code): Update LIVE and reg pressure information.
        (calculate_bb_reg_pressure): Initialize live_in and backup.

        Backport from mainline r193425
        2012-11-12  Bin Cheng  <bin.ch...@arm.com>

        * gcc.dg/hoist-register-pressure-3.c: New test.