aarch64: Tweak insn sequence code

aarch64: Tweak insn sequence code

libopcodes has some code to check constraints across sequences
of consecutive instructions.  It was added to support MOVPRFX
sequences but is going to be useful for the Armv8.8-A MOPS
feature as well.

Currently the structure has one field to record the instruction
that started a sequence and another to record the remaining
instructions in the sequence.  It's more convenient for the
MOPS code if we put the instructions into a single array instead.

No functional change intended.

include/
	* opcode/aarch64.h (aarch64_instr_sequence): Replace num_insns
	and current_insns with num_added_insns and num_allocated_insns.

opcodes/
	* aarch64-opc.c (add_insn_to_sequence): New function.
	(init_insn_sequence): Update for new aarch64_instr_sequence layout.
	Add the first instruction to the inst array.
	(verify_constraints): Update for new aarch64_instr_sequence layout.
	Don't add the last instruction to the array.