s390: More vec-perm-const cases.

s390: More vec-perm-const cases.

s390 missed constant vector permutation cases based on the vector pack
instruction or changing the size of the vector elements during vector
merge.  This enables some more patterns that do not need to load a
constant vector for permutation.

gcc/ChangeLog:

	* config/s390/s390.cc (expand_perm_with_merge): Add size change cases.
	(expand_perm_with_pack): New function.
	(vectorize_vec_perm_const_1): Wire up new function.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/vec-perm-merge-1.c: New test.
	* gcc.target/s390/vector/vec-perm-pack-1.c: New test.

Signed-off-by: Juergen Christ <jchrist@linux.ibm.com>