[AArch64] Pass number of units to aarch64_expand_vec_perm(_const)

[AArch64] Pass number of units to aarch64_expand_vec_perm(_const)

This patch passes the number of units to aarch64_expand_vec_perm
and aarch64_expand_vec_perm_const, which avoids a to_constant ()
once GET_MODE_NUNITS is variable.

2017-11-06  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* config/aarch64/aarch64-protos.h (aarch64_expand_vec_perm)
	(aarch64_expand_vec_perm_const): Take the number of units too.
	* config/aarch64/aarch64.c (aarch64_expand_vec_perm)
	(aarch64_expand_vec_perm_const): Likewise.
	* config/aarch64/aarch64-simd.md (vec_perm_const<mode>)
	(vec_perm<mode>): Update accordingly.

Reviewed-by: James Greenhalgh <james.greenhalgh@arm.com>
Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254469