* Removed unused classes: the rd field is always mentioned as the last reg.

* Added new classes which start building from rs1, adding rs2, and then rd.
* Fixed order of operands in classes 3.11, 3.12, 3.16, and 3.17 .
* Fixed comments to reflect Real Life (tm).
* Removed "don't care" commented out assignments and dead classes (#if 0).

llvm-svn: 6560