patch for PR65862

[Vladimir Makarov <vmakarov at redhat dot com>]

The following patch is for solution of PR65862

   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65862

In some cases, calculations of reg class costs are not enough to choose 
the right class.  For example, in some cases you don't want to start fp 
unit for energy saving considerations if you don't really need it.  Such 
case can occur when we have non-floating point mode pseudo and 
calculation costs in ira says that it would be profitable to put it in a 
floating point register.

The full solution will need to add a hook to mips.c.  I hope mips 
maintainers will do that soon.

The patch was bootstrapped on x86-64.

Committed as rev. 223202.

2015-05-14  Vladimir Makarov  <vmakarov@redhat.com>

        PR rtl-optimization/65862
        * target.def (ira_change_pseudo_allocno_class): New hook.
        * targhooks.c (default_ira_change_pseudo_allocno_class): Default
        value of the hook.
        * targhooks.h (default_ira_change_pseudo_allocno_class): New
        extern
        * doc/tm.texi.in (TARGET_IRA_CHANGE_PSEUDO_ALLOCNO_CLASS): Add the
        hook.
        * ira-costs.c (find_costs_and_classes): Call the hook and change
        classes when it is necessary.
        * doc/tm.texi: Update.