Hi Joern,

This supercedes my last patch.  All I'm doing in this one is
expand_opcode, implementing some of the changes we discussed
(to pave the way for the sh4a patch to come).  It's much
simpler, and fewer lines of code.

It now accepts either 2 or 4 bits at a time (except for ones
and zeroes, which accept an arbitrary sequence).  The sub-
cases such as 'N' and 'x' are moved out to the outer switch
(so its flatter and simpler).  The "shift" parameter is gone
(which required that the movx/movy expansion be moved into
the post-processing pass in ppi_moves).  "ddd0" becomes
"eeee" (even numbered register), and I don't check it at runtime.

The XY/xy stuff will come in the next patch.

Michael