[AArch64/GCC][17/N] Optimize prologue when there is no frame pointe

[Jiong Wang]

Under new pro/epi code, we could also utilize our store write-back to optimize
stack adjustment when there is no frame pointer.

* if there is candidate reg pair and adjustment amount is less than 512 then we
could use aarch64's paired store write-back.
* if there is only a single candidate reg and adjustment amount is less than 256,
we could use aarch64's single store write-back.
* otherwise use explictly subtraction to finish stack adjustment.
     
   Improved testcases:
     
     gcc.target/aarch64/test_frame_1.c
     gcc.target/aarch64/test_frame_10.c
     gcc.target/aarch64/test_frame_2.c
     gcc.target/aarch64/test_frame_4.c
     gcc.target/aarch64/test_frame_6.c
     gcc.target/aarch64/test_frame_7.c
     gcc.target/aarch64/test_frame_8.c
     gcc.target/aarch64/test_fp_attribute_1.c

ok for install?

gcc/
   * config/aarch64/aarch64.c (aarch64_pushwb_single_reg): New function.
   (aarch64_expand_prologue): Optimize prologue when !frame_pointer_needed.

gcc/testsuite/
   * gcc.target/aarch64/test_frame_1.c: Match optimized instruction sequences.
   * gcc.target/aarch64/test_frame_10.c: Likewise.
   * gcc.target/aarch64/test_frame_2.c: Likewise.
   * gcc.target/aarch64/test_frame_4.c: Likewise.
   * gcc.target/aarch64/test_frame_6.c: Likewise.
   * gcc.target/aarch64/test_frame_7.c: Likewise.
   * gcc.target/aarch64/test_frame_8.c: Likewise.
   * gcc.target/aarch64/test_fp_attribute_1.c: Likewise.
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