[AArch64] Add V8_2_INSN macro

[AArch64] Add V8_2_INSN macro

For consistency with the previous two patches, this one
adds a macro for the two ARMv8.2 table entries.  Both table
entries need a non-null aarch64_op field.

I haven't added macros for the RAS and STAT_PROFILE entries
since there's only one of each.  The series isn't getting
rid of braced entries altogether, so I've only looked at
replacing things that occur more than once.

opcodes/
	* aarch64-tbl.h (V8_2_INSN): New macro.
	(aarch64_opcode_table): Use it.