IBM Z: Implement TARGET_VECTORIZE_VEC_PERM_CONST for vector merge

IBM Z: Implement TARGET_VECTORIZE_VEC_PERM_CONST for vector merge

This patch implements the TARGET_VECTORIZE_VEC_PERM_CONST in the IBM Z
backend. The initial implementation only exploits the vector merge
instruction but there is more to come.

gcc/ChangeLog:

	* config/s390/s390.c (MAX_VECT_LEN): Define macro.
	(struct expand_vec_perm_d): Define struct.
	(expand_perm_with_merge): New function.
	(vectorize_vec_perm_const_1): New function.
	(s390_vectorize_vec_perm_const): New function.
	(TARGET_VECTORIZE_VEC_PERM_CONST): Define target macro.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/perm-merge.c: New test.
	* gcc.target/s390/vector/vec-types.h: New test.