[AArch64] Improve SVE handling of single-vector permutes

[AArch64] Improve SVE handling of single-vector permutes

aarch64_vectorize_vec_perm_const was failing to set one_vector_p
if the permute had only a single input.  This in turn was hiding
a problem in the SVE TBL handling: it accepted single-vector
variable-length permutes, but sent them through the general
two-vector aarch64_expand_sve_vec_perm, which is only set up
to handle constant-length permutes.

2018-08-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_evpc_sve_tbl): Fix handling
	of single-vector TBLs.
	(aarch64_vectorize_vec_perm_const): Set one_vector_p when only
	one input is given.

gcc/testsuite/
	* gcc.dg/vect/no-vfa-vect-depend-2.c: Remove XFAIL.
	* gcc.dg/vect/no-vfa-vect-depend-3.c: Likewise.
	* gcc.dg/vect/pr65947-13.c: Update for vect_fold_extract_last.
	* gcc.dg/vect/pr80631-2.c: Likewise.

From-SVN: r263804