This patch simplify aarch64_expand_prologue using our new
helper functions. All ad-hoc code for saving FP/LR are removed.

*no functional change*

OK to install?

thanks.

gcc/
   * config/aarch64/aarch64.c (aarch64_gen_storewb_pair): New helper function.
   (aarch64_pushwb_pair_reg): Likewise.
   (aarch64_expand_prologue): Simplify code using new helper functions.