[NDS32] Refine CASE_VECTOR_SHORTEN_MODE to function.

gcc/
	* config/nds32/nds32-protos.h (nds32_case_vector_shorten_mode):
	Declare.
	* config/nds32/nds32.c (nds32_case_vector_shorten_mode): New function.
	* config/nds32/nds32.h (CASE_VECTOR_SHORTEN_MODE): Modify.

Co-Authored-By: Chung-Ju Wu <jasonwucj@gmail.com>

From-SVN: r259550