better handling of unknown operands

[Ben Elliston]

I observed some undesirable behavior in cgen this morning.  Before I
set out to fix it, I'd like to discuss possible solutions.  When
defining the assembly syntax string for an instruction, I typo'd the
following:

	" .. $(rs) .."
rather than:
	" .. ($rs) .."

It looks like the parser encountered the `$', advanced to the `(' and
stopped.  In the generated code, I see a reference to OP () which is
wrong.

More generally, shouldn't cgen complain about any operand that appears
in the syntax string but not the field format list, or vice-versa?

Ben