Remove bogus initialization of the PPC and Hexagon SelectionDAGISel

subclasses. These are not passes proper. We don't support registering
them, they can't be constructed with default arguments, and the ID is
actually in a base class.

Only these two targets even had any boiler plate to try to do this, and
it had to be munged out of the INITIALIZE_PASS macros to work. What's
worse, the boiler plate has rotted and the "name" of the pass is
actually the description string now!!! =/ All of this is completely
unnecessary. No other target bothers, and nothing breaks if you don't
initialize them because CodeGen has an entirely separate initialization
path that is somewhat more durable than relying on the implicit
initialization the way the 'opt' tool does for registered passes.

llvm-svn: 271650