Require some enhancement in CGEN for decoder of disassember

[Jie Zhang]

I encounter a problem when I'm using CGEN to port binutils.
In my target, there is a rather strange 4-bit field.
There are two insns, A and B. This field is operand in insn A,
but opcode for insn B.
A and B can be distinguished only by this field.
In insn A, the value of the operand can range from 0 through 14.
If this field is 15, then the insn is B.
If I put the description of insn A before B, insn B is always
disassembled as insn A with the wrong operand value 15.
I can solve this problem by putting the description fo insn B before A,
but this is not a good solution.
I wish CGEN can deal with this issue.

-Jie Zhang