aarch64: Use local frame vars in shrink-wrapping code

aarch64: Use local frame vars in shrink-wrapping code

aarch64_layout_frame uses a shorthand for referring to
cfun->machine->frame:

  aarch64_frame &frame = cfun->machine->frame;

This patch does the same for some other heavy users of the structure.
No functional change intended.

gcc/
	* config/aarch64/aarch64.cc (aarch64_save_callee_saves): Use
	a local shorthand for cfun->machine->frame.
	(aarch64_restore_callee_saves, aarch64_get_separate_components):
	(aarch64_process_components): Likewise.
	(aarch64_allocate_and_probe_stack_space): Likewise.
	(aarch64_expand_prologue, aarch64_expand_epilogue): Likewise.
	(aarch64_layout_frame): Use existing shorthand for one more case.