s390: Recognize reverse/element swap permute patterns.

s390: Recognize reverse/element swap permute patterns.

This adds functions to recognize reverse/element swap permute patterns
for vler, vster as well as vpdi and rotate.

gcc/ChangeLog:

	* config/s390/s390.cc (expand_perm_with_vpdi): Recognize swap pattern.
	(is_reverse_perm_mask): New function.
	(expand_perm_with_rot): Recognize reverse pattern.
	(expand_perm_with_vstbrq): New function.
	(expand_perm_with_vster): Use vler/vster for element reversal on z15.
	(vectorize_vec_perm_const_1): Use.
	(s390_vectorize_vec_perm_const): Add expand functions.
	* config/s390/vx-builtins.md: Prefer vster over vler.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/vperm-rev-z14.c: New test.
	* gcc.target/s390/vector/vperm-rev-z15.c: New test.
	* gcc.target/s390/zvector/vec-reve-store-byte.c: Adjust test
	expectation.