Avoid GCC 4.1 build failure in fold-const.c

Avoid GCC 4.1 build failure in fold-const.c

We had:

	      tree t = fold_vec_perm (type, arg1, arg2,
				      vec_perm_indices (sel, 2, nelts));

where fold_vec_perm takes a const vec_perm_indices &.  GCC 4.1 apparently
required a public copy constructor:

gcc/vec-perm-indices.h:85: error: 'vec_perm_indices::vec_perm_indices(const vec_perm_indices&)' is private
gcc/fold-const.c:11410: error: within this context

even though no copy should be made here.  This patch tries to work
around that by constructing the vec_perm_indices separately.

2018-01-16  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* fold-const.c (fold_ternary_loc): Construct the vec_perm_indices
	in a separate statement.

From-SVN: r256740