aarch64: Add an abstraction for vector base addresses

aarch64: Add an abstraction for vector base addresses

In the upcoming SVE2.1 svld1q and svst1q intrinsics, the relationship
between the base vector and the data vector differs from existing
gather/scatter intrinsics.  This patch adds a new abstraction to
handle the difference.

gcc/
	* config/aarch64/aarch64-sve-builtins.h
	(function_shape::vector_base_type): New member function.
	* config/aarch64/aarch64-sve-builtins.cc
	(function_shape::vector_base_type): Likewise.
	(function_resolver::resolve_sv_displacement): Use it.
	(function_resolver::resolve_gather_address): Likewise.