PR83675: Restore TARGET_VIS2 check for SPARC vec_perm_const

I'd missed a TARGET_VIS2 condition when replacing vec_perm_const_optab
with TARGET_VECTORIZE_VEC_PERM_CONST in r256093.

2018-01-04  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR target/83675
	* config/sparc/sparc.c (sparc_vectorize_vec_perm_const): Require
	TARGET_VIS2.

From-SVN: r256229

gcc/ChangeLog

+2018-01-04  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR target/83675
+	* config/sparc/sparc.c (sparc_vectorize_vec_perm_const): Require
+	TARGET_VIS2.
+
 2018-01-04  Uros Bizjak  <ubizjak@gmail.com>
 
 	PR target/83628

gcc/config/sparc/sparc.c

@@ -12824,6 +12824,9 @@ static bool
 sparc_vectorize_vec_perm_const (machine_mode vmode, rtx target, rtx op0,
 				rtx op1, const vec_perm_indices &sel)
 {
+  if (!TARGET_VIS2)
+    return false;
+
   /* All permutes are supported.  */
   if (!target)
     return true;