Fix permute handling when vectorising scatters

As mentioned in https://gcc.gnu.org/ml/gcc-patches/2017-11/msg01575.html ,
the scatter handling in vectorizable_store seems to be dead code at the
moment.  Enabling it with the vect_analyze_data_ref_access part of
that patch triggered an ICE in the avx512f-scatter-*.c tests (which
previously didn't use scatters).  The problem was that the NARROW
and WIDEN handling uses permute_vec_elements to marshal the inputs,
and permute_vec_elements expected the lhs of the stmt to be an SSA_NAME,
which of course it isn't for stores.

This patch makes permute_vec_elements create a fresh variable in this case.

2018-01-09  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vect-stmts.c (permute_vec_elements): Create a fresh variable
	if the destination isn't an SSA_NAME.

From-SVN: r256383

gcc/ChangeLog

+2018-01-09  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	* tree-vect-stmts.c (permute_vec_elements): Create a fresh variable
+	if the destination isn't an SSA_NAME.
+
 2018-01-09  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/83668

gcc/tree-vect-stmts.c

@@ -6585,7 +6585,11 @@ permute_vec_elements (tree x, tree y, tree mask_vec, gimple *stmt,
   tree perm_dest, data_ref;
   gimple *perm_stmt;
 
-  perm_dest = vect_create_destination_var (gimple_get_lhs (stmt), vectype);
+  tree scalar_dest = gimple_get_lhs (stmt);
+  if (TREE_CODE (scalar_dest) == SSA_NAME)
+    perm_dest = vect_create_destination_var (scalar_dest, vectype);
+  else
+    perm_dest = vect_get_new_vect_var (vectype, vect_simple_var, NULL);
   data_ref = make_ssa_name (perm_dest);
 
   /* Generate the permute statement.  */