[binutils][aarch64] New sve_size_tsz_bhs iclass.

[binutils][aarch64] New sve_size_tsz_bhs iclass.

Add sve_size_tsz_bhs iclass needed for sqxtnb and similar instructions.
This iclass encodes one of three variants by the most significant bit
set in a 3-bit value where only one bit may be set.

include/ChangeLog:

2019-05-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* opcode/aarch64.h (enum aarch64_insn_class): Add sve_size_tsz_bhs
	iclass.

opcodes/ChangeLog:

2019-05-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* aarch64-asm.c (aarch64_encode_variant_using_iclass): Handle
	sve_size_tsz_bhs iclass encode.
	* aarch64-dis.c (aarch64_decode_variant_using_iclass): Handle
	sve_size_tsz_bhs iclass decode.