Add "mverilog" Compiler Option, Compiler Fixes

This adds "mverilog" to the "--compiler" command line option. This
will run the MinimumVerilogCompiler.

This additionally fixes the MinimumVerilogCompiler such that
DeadCodeElimination will not be run (it's not supposed to be). This is
done by adding a MinimumVerilogEmitter, subclassing VerilogVerilog,
that strips the DeadCodeElimination step from its parent.

Additionally, BlackBoxSourceHelper is removed from the
MinimumVerilogCompiler since this will be run by the VerilogEmitter
already.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>