[Bug tree-optimization/65791] New: Postpone expand_ifn_va_arg till after optimize_va_list_gpr_fpr_size

["vries at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65791

            Bug ID: 65791
           Summary: Postpone expand_ifn_va_arg till after
                    optimize_va_list_gpr_fpr_size
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org

After the fix for PR64950 - postpone expanding va_arg till pass_stdarg, there's
this TODO pass_stdarg::execute:
...
unsigned int
pass_stdarg::execute (function *fun)
{
  /* TODO: Postpone expand_ifn_va_arg till after
     optimize_va_list_gpr_fpr_size.  */
  expand_ifn_va_arg (fun);

  if (flag_stdarg_opt
      /* This optimization is only for stdarg functions.  */
      && fun->stdarg != 0)
    optimize_va_list_gpr_fpr_size (fun);

  return 0;
}
...

Essentially, we want to rewrite optimize_va_list_gpr_fpr_size to work on
ifn_va_arg.