Fixes for addv and xtn2 instructions.

Fixes for addv and xtn2 instructions.

	sim/aarch64/
	* simulator.c (do_vec_ADDV): Call aarch64_set_vec_u64 instead of
	aarch64_set_reg_u64.  In case 2, call HALT_UNALLOC if not full.  In
	case 3, call HALT_UNALLOC unconditionally.
	(do_vec_XTN): Delete shifts.  In case 2, change index from i + 4 to
	i + 2.  Delete if on bias, change index to i + bias * X.

	sim/testsuite/sim/aarch64/
	* addv.s: New.
	* xtn.s: New.