AArch64: Wire through instr_sequence

AArch64: Wire through instr_sequence

This patch introduces aarch64_instr_sequence which is a structure similar to IT
blocks on Arm in order to track instructions that introduce a constraint or
dependency on instruction 1..N positions away from the instruction that opened
the block.

The struct is also wired through to the locations that require it.

gas/

	* config/tc-aarch64.c (now_instr_sequence):
	(*insn_sequence, now_instr_sequence): New.
	(output_operand_error_record, do_encode): Add insn_sequence.
	(md_assemble): Update insn_sequence.
	(try_to_encode_as_unscaled_ldst, fix_mov_imm_insn, fix_insn):
	Pass insn_sequence.
	* config/tc-aarch64.h (struct aarch64_segment_info_type):
	Add insn_sequence.

include/

	* opcode/aarch64.h (struct aarch64_instr_sequence): New.
	(aarch64_opcode_encode): Use it.

opcodes/

	* aarch64-asm.c (aarch64_opcode_encode): Add insn_sequence.
	* aarch64-dis.c (insn_sequence): New.