Change permute index type to unsigned short

Change permute index type to unsigned short

This patch changes the element type of (auto_)vec_perm_indices from
unsigned char to unsigned short.  This is needed for fixed-length
2048-bit SVE.  (SVE is variable-length by default, but it's possible
to ask for specific vector lengths if you want to.)

2017-09-25  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* target.h (vec_perm_indices): Use unsigned short rather than
	unsigned char.
	(auto_vec_perm_indices): Likewise.
	* config/aarch64/aarch64.c (aarch64_vectorize_vec_perm_const_ok):
	Use unsigned int rather than unsigned char.
	* config/arm/arm.c (arm_vectorize_vec_perm_const_ok): Likewise.

From-SVN: r253150