Optimized Low Firrtl Emitter should not emit skip statements

[ekiwi]

Checklist

• Did you specify the current behavior?
• Did you specify the expected behavior?
• Did you provide a code example showing the problem?

What is the current behavior?

The current low-opt emitter generates a bunch of skip statements.

What is the expected behavior?

It would be nice if we could filter out the skip statements before serializing.

Steps to Reproduce

On the latest snapshot of chisel, something like:

(new ChiselStage).emitFirrtl(new MyDesign(), Array("-E", "low-opt"))