aarch64: Add a result_mode helper function

SME will add more intrinsics whose expansion code requires
the mode of the function return value.  This patch adds an
associated helper routine.

gcc/
	* config/aarch64/aarch64-sve-builtins.h
	(function_expander::result_mode): New member function.
	* config/aarch64/aarch64-sve-builtins-base.cc
	(svld234_impl::expand): Use it.
	* config/aarch64/aarch64-sve-builtins.cc
	(function_expander::get_reg_target): Likewise.