Add two new frame fields:

   * cfun->machine->frame.wb_candidate1
   * cfun->machine->frame.wb_candidate2
    
* wb_candidate1 to record the first reg index which
   could carry write-back.
* wb_candidate2 to record the second reg index which
   could form reg pair with
* wb_candidate1 to carry write-back when doing paired
   load/store.

They are useful in later pro/epi optimization.

*no functional change*

ok to install?

thanks.

gcc/
   * config/aarch64/aarch64.h (frame): New fields "wb_candidate1" and "wb_candidate2".
   * config/aarch64/aarch64.c (aarch64_layout_frame): Calcualte new added fields.