aarch64_opcode_table changes for BZ 19722

[Szabolcs Nagy]

Nick, I'm trying to understand your changes in

https://sourceware.org/bugzilla/show_bug.cgi?id=19722
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=4bd13cde17a27c342b79b72bde9ef8e1b5373344

with the new verifier field some

 {a,b,c,d},

table entries were changed to

 {a,b,c,d,NULL},

others to

 c_INS(a,b,d),

the goal of those c_INS macros is not clear to me,
is it just to align the opcode table columns?

i'd like to fiddle with the opcode table too, so
let me know if these have some special semantics.