Use valid_for_const_vector_p instead of CONSTANT_P

Use valid_for_const_vector_p instead of CONSTANT_P

This patch makes the VEC_SERIES code use valid_for_const_vector_p
instead of CONSTANT_P, to match what we already do for VEC_DUPLICATE.
This showed up as a failure in gcc.c-torture/execute/pr28982b.c for -m32
on x86_64-linux-gnu after later patches.

2017-12-28  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* emit-rtl.c (gen_const_vec_series): Use valid_for_const_vector_p
	instead of CONSTANT_P.
	(gen_vec_series): Likewise.
	* simplify-rtx.c (simplify_binary_operation_1): Likewise.

From-SVN: r256023