The "new" VEC_PERM_EXPR handling makes it easy to support bswap
for variable-length vectors.

2018-08-24  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vectorizable_bswap): Handle variable-length
	vectors.

gcc/testsuite/
	* gcc.target/aarch64/sve/bswap_1.c: New test.
	* gcc.target/aarch64/sve/bswap_2.c: Likewise.
	* gcc.target/aarch64/sve/bswap_3.c: Likewise.

From-SVN: r263833