This patch refactor the inner loop out into "aarch64_next_callee_save",
and simplify the code logic and reduce indent.

*no functional change*

ok to install?

thanks.

gcc/
   * config/aarch64/aarch64.c (aarch64_next_callee_save): New function.
   (aarch64_save_or_restore_fprs): Use aarch64_next_callee_save.
   (aarch64_save_or_restore_callee_save_registers): Likewise.