arm: Factorize several occurrences of the same code into reg_needs_saving_p

arm: Factorize several occurrences of the same code into reg_needs_saving_p

The same code pattern occurs in several functions, so it seems cleaner
to move it into a dedicated function.

2020-05-14  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/arm.c (reg_needs_saving_p): New function.
	(use_return_insn): Use reg_needs_saving_p.
	(arm_get_vfp_saved_size): Likewise.
	(arm_compute_frame_layout): Likewise.
	(arm_save_coproc_regs): Likewise.
	(thumb1_expand_epilogue): Likewise.
	(arm_expand_epilogue_apcs_frame): Likewise.
	(arm_expand_epilogue): Likewise.