[AArch64] Implement vector float->double widening and double->float narrowing.

[AArch64] Implement vector float->double widening and double->float  narrowing.

gcc/
	* config/aarch64/aarch64-simd-builtins.def (vec_unpacks_hi_): New.
	(float_truncate_hi_): Likewise.
	(float_extend_lo_): Likewise.
	(float_truncate_lo_): Likewise.
	* config/aarch64/aarch64-simd.md (vec_unpacks_lo_v4sf): New.
	(aarch64_float_extend_lo_v2df): Likewise.
	(vec_unpacks_hi_v4sf): Likewise.
	(aarch64_float_truncate_lo_v2sf): Likewise.
	(aarch64_float_truncate_hi_v4sf): Likewise.
	(vec_pack_trunc_v2df): Likewise.
	(vec_pack_trunc_df): Likewise.

From-SVN: r198400