aarch64: Add support for optional operand pairs

aarch64: Add support for optional operand pairs

Two of the instructions added by the `+d128' architectural extension
add the flexibility to have two optional operands.  Prior to the
addition of the `tlbip' and `sysp' instructions, no mnemonic allowed
more than one such optional operand.

With `tlbip' as an example, some TLBIP instruction names do not allow
for any optional operands, while others allow for both to be optional.
In the latter case, it is possible that either the second operand
alone is omitted or both operands are omitted.
Therefore, a considerable degree of flexibility needed to be added to
the way operands were parsed.  It was, however, possible to achieve
this with relatively few changes to existing code.

it is noteworthy that opcode flags specifying the optional operand
number are non-orthogonal. For example, we have:

       #define F_OPD1_OPT (2 << 12) : 0b10 << 12
       #define F_OPD2_OPT (3 << 12) : 0b11 << 12

such that by virtue of the observation that

       (F_OPD1_OPT | F_OPD2_OPT) == F_OPD2_OPT

it is impossible to mark both operands 1 and 2 as optional for an
instruction and it is assumed that a maximum of 1 operand can ever be
optional.  This is not overly-problematic given that, for optional
pairs, the second optional operand is always found immediately after
the first.  Thus, it suffices for us to flag that there is a second
optional operand.  With this fact, we can infer its position in the
mnemonic from the position of the first (e.g. if the second operand in
the mnemonic is optional, we know the third is too).  We therefore
define the `F_OPD_PAIR_OPT' flag and calculate its position in the
mnemonic from the value encoded by the `F_OPD<n>_OPT' flag.

Another observation is that there is a tight coupling between default
values assigned to the two registers when one (or both) are omitted
from the mnemonic.  Namely, if Xt1 has a value of 0x1f (the zero
register is specified), Xt2 defaults to the same value, otherwise Xt2
will be assigned Xt + 1.  This meant that where you have default value
validation, in checking the second optional operand's value, it is
also necessary to look at the value assigned to the
previously-processed operand value before deciding its validity. Thus
`process_omitted_operand' needs not only access to its `operand'
argument, but also to the global `inst' struct.