aarch64:sve: Use make_ssa_name instead of create_tmp_var in the folder

aarch64:sve: Use make_ssa_name instead of create_tmp_var in the folder

Currently gimple_folder::convert_and_fold calls create_tmp_var; that
means while in ssa form, the pass which calls fold_stmt will always
have to update the ssa (via TODO_update_ssa or otherwise).  This seems
not very useful since we know that this will always be a ssa name, using
make_ssa_name instead is better and don't need to depend on the ssa updater.
Plus this should have a small compile time performance and memory usage
improvement too since this uses an anonymous ssa name rather than creating a
full decl for this.

Changes since v1:
* Use make_ssa_name instead of create_tmp_reg_or_ssa_name, anonymous ssa
  names are allowed early on in gimple too.

Built and tested on aarch64-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/aarch64-sve-builtins.cc: Include value-range.h and tree-ssanames.h
	(gimple_folder::convert_and_fold): Use make_ssa_name
	instead of create_tmp_var for the temporary. Add comment about callback argument.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>