RISC-V: Export functions as global extern preparing for dynamic LMUL patch use

RISC-V: Export functions as global extern preparing for dynamic LMUL patch use

Notice those functions need to be use by COST model for dynamic LMUL use.
Extract as a single patch and committed.

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (lookup_vector_type_attribute): Export global.
	(get_all_predecessors): New function.
	(get_all_successors): Ditto.
	* config/riscv/riscv-v.cc (get_all_predecessors): Ditto.
	(get_all_successors): Ditto.
	* config/riscv/riscv-vector-builtins.cc (sizeless_type_p): Export global.
	* config/riscv/riscv-vsetvl.cc (get_all_predecessors): Remove it.