s390: Add LEN_LOAD/LEN_STORE support.

This patch adds LEN_LOAD/LEN_STORE support for z13 and newer.
It defines a bias value of -1 and implements the LEN_LOAD and LEN_STORE
optabs.

Add vll/vstl testcases adapted from Power.

Also change expectations for SLP testcases with more than one rgroup.

gcc/ChangeLog:

	* config/s390/predicates.md (vll_bias_operand): Add -1 bias.
	* config/s390/s390.cc (s390_option_override_internal): Make
	partial vector usage the default from z13 on.
	* config/s390/vector.md (len_load_v16qi): Add.
	(len_store_v16qi): Add.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/s390.exp: Add partial subdirectory.
	* gcc.target/s390/vector/vec-nopeel-2.c: Change test
	expectation.
	* lib/target-supports.exp: Add s390.
	* gcc.target/s390/vector/partial/s390-vec-length-1.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-2.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-3.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-7.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-1.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-2.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-3.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-7.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-1.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-2.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-3.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-7.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-1.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-2.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-3.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-7.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-1.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-2.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-3.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-7.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-run-1.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-run-2.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-run-3.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-run-7.h: New test.
	* gcc.target/s390/vector/partial/s390-vec-length-small.c: New test.
	* gcc.target/s390/vector/partial/s390-vec-length.h: New test.