RFA: patch solving one IRA problem reported in PR37243

[Jeff Law]

Vladimir Makarov wrote:
> The following patch solves IRA regression on pr36222 and p36246 of gcc
> testsuite.  The analysis of the problem could be found in
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37243
>
> The patch has been bootstrapped sucessfully on x86_64.  It also
> improves SPEC results at least for x86.
>
> 2008-08-29  Vladimir Makarov  <vmakarov@redhat.com>
>
>    PR rtl-opt/37243
>
>    * ira-conflicts.c (REG_SUBREG_P, go_through_subreg): New.
>    (process_regs_for_copy): Process subregs.  Refine check when cost
>    is taken into account in ira-costs.c.
>    (process_reg_shuffles): Use REG_SUBREG_P.
>    (add_insn_allocno_copies): Ditto.  Ignore modes.
>
>    * ira-color.c (conflict_allocno_vec): New.
>    (COST_HOP_DIVISOR): New macro.
>    (update_copy_costs_1): Use it.
>    (update_conflict_hard_regno_costs): New function.
>    (assign_hard_reg): Use it.
>    (ira_color): Allocate and free conflict_allocno_vec.
>   
This got me thinking about a long standing poor code problem on the H8 
(and likely other processors).  Can IRA "tie" a dying source operand to 
a destination operand when the source & destination operands have 
different modes?

This comes up with the zero/sign extension patterns on some targets.  
The source & destination are required to match, but they have different 
modes.  This case was never handled well by the old allocator resulting 
in some obviously bad register allocations.

Jeff