Handle poly_int vector sizes in get_vec_alignment_for_array_type

Handle poly_int vector sizes in get_vec_alignment_for_array_type

get_vectype_for_scalar_type returns a variable-length vector type
for SVE, whereas get_vec_alignment_for_array_type assumed it would
always be an INTEGER_CST.

This is needed to build libstdc++-v3/src/closures.cc for SVE
(and probably many other places besides -- this was just the
first hit).

2018-01-12  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vectorizer.c (get_vec_alignment_for_array_type): Handle
	polynomial type sizes.

From-SVN: r256586