aarch64: Split out aarch64_adjust_body_cost_sve

aarch64: Split out aarch64_adjust_body_cost_sve

This patch splits the SVE-specific part of aarch64_adjust_body_cost
out into its own subroutine, so that a future patch can call it
more than once.  I wondered about using a lambda to avoid having
to pass all the arguments, but in the end this way seemed clearer.

gcc/
	* config/aarch64/aarch64.c (aarch64_adjust_body_cost_sve): New
	function, split out from...
	(aarch64_adjust_body_cost): ...here.