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