Allow VEC_PERM_EXPR folding to fail

tree-ssa-forwprop.c was asserting that a VEC_PERM_EXPR fold on three
VECTOR_CSTs would always succeed, but it's possible for it to fail
with variable-length vectors.

2017-12-22  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-ssa-forwprop.c (is_combined_permutation_identity): Allow
	the VEC_PERM_EXPR fold to fail.

From-SVN: r256257

gcc/ChangeLog

+2018-01-04  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	* tree-ssa-forwprop.c (is_combined_permutation_identity): Allow
+	the VEC_PERM_EXPR fold to fail.
+
 2018-01-04  Jakub Jelinek  <jakub@redhat.com>
 
 	PR debug/83585

gcc/tree-ssa-forwprop.c

@@ -1865,7 +1865,8 @@ is_combined_permutation_identity (tree mask1, tree mask2)
   gcc_checking_assert (TREE_CODE (mask1) == VECTOR_CST
 		       && TREE_CODE (mask2) == VECTOR_CST);
   mask = fold_ternary (VEC_PERM_EXPR, TREE_TYPE (mask1), mask1, mask1, mask2);
-  gcc_assert (TREE_CODE (mask) == VECTOR_CST);
+  if (mask == NULL_TREE || TREE_CODE (mask) != VECTOR_CST)
+    return 0;
 
   if (!VECTOR_CST_NELTS (mask).is_constant (&nelts))
     return 0;