aarch64: Move and generalise vect_all_same

aarch64: Move and generalise vect_all_same

The fix for PR106329 needs a way of testing for a ptrue of a particular
element size.  We already had such a function for svlast, so this patch
moves it to common code and generalises it to work with all kinds of
vectors.

gcc/
	* config/aarch64/aarch64-sve-builtins.h (vector_cst_all_same): Declare.
	* config/aarch64/aarch64-sve-builtins.cc (vector_cst_all_same): New
	function, a generalized replacement of...
	* config/aarch64/aarch64-sve-builtins-base.cc
	(svlast_impl::vect_all_same): ...this.
	(svlast_impl::fold): Update accordingly.