Fix code generation error that resulted in a multi-argument NEGQ

doasm: notfound ft=13 tt=13 00134 NEGQ	AX, AX 13 13

Change-Id: I8b712d21a5523eccbae1f33ccea417844c27073e
Reviewed-on: https://go-review.googlesource.com/12869Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>