opcodes: aarch64: enforce checks on subclass flags in aarch64-gen.c

[New in V4]

Enforce some checks on the newly added subclass flags:
  - If a subclass is set of one insn of an iclass, every insn of that
    iclass must have non-zero subclass field.
  - For all other iclasses, the subclass bits are zero for all insns.

include/
        * opcode/aarch64.h (enum aarch64_insn_class): Identify the
	maximum iclass enum value.

opcodes/
        * aarch64-gen.c (iclass_has_subclasses_p): New array of bool.
        (read_table): Enforce checks on subclass flags.