Fix permute handling when vectorising scatters

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