aarch64: use an enumeration for operand indices.

The generated aarch64 operand tables use index values into an array.  But if
the table of operands is modified by inserting a new operand into the middle
of the table, *all* the index values can change, leading to a lot of
churn in the generated output.

include/opcode/aarch64.h already provides an enumeration for the operands,
so make use of that instead of printing out the raw index values.