Check whether a vector of QIs can store all indices

Check whether a vector of QIs can store all indices

The patch to remove the vec_perm_const optab checked whether replacing
a constant permute with a variable permute is safe, or whether it might
truncate the indices.  This patch adds a corresponding check for whether
variable permutes can be lowered to QImode-based permutes.

2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* optabs-query.c (can_vec_perm_var_p): Check whether lowering
	to qimode could truncate the indices.
	* optabs.c (expand_vec_perm_var): Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256094 138bc75d-0d04-0410-961f-82ee72b054a4