aarch64: Commonise some folding code

aarch64: Commonise some folding code

Add an aarch64_sve::gimple_folder helper for folding calls
to integer constants.  SME will make more use of this.

gcc/
	* config/aarch64/aarch64-sve-builtins.h
	(gimple_folder::fold_to_cstu): New member function.
	* config/aarch64/aarch64-sve-builtins.cc
	(gimple_folder::fold_to_cstu): Define.
	* config/aarch64/aarch64-sve-builtins-base.cc
	(svcnt_bhwd_impl::fold): Use it.