Allow VEC_PERM_EXPR folding to fail

[Richard Sandiford]

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.

Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu.
Also tested by comparing the before and after assembly output for at
least one target per CPU directory.  OK to install?

Richard


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

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

Index: gcc/tree-ssa-forwprop.c
===================================================================
--- gcc/tree-ssa-forwprop.c	2018-01-03 21:42:44.571646705 +0000
+++ gcc/tree-ssa-forwprop.c	2018-01-04 10:00:03.582673530 +0000
@@ -1865,7 +1865,8 @@ is_combined_permutation_identity (tree m
   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;