add IOFF inference for qlf_k6n10f

[nakengelhardt]

What are the reasons/motivation for this change?

There are registered I/Os available in the qlf_k6n10f architecture, using them saves fabric FFs.

Explain how this is achieved.

If a dffsre or sdffsre cell in the top module fulfils the following criteria:

• the set, reset, and enable ports are unused
• the D port is connected to an input signal or the Q port is connected to an output signal
• the signal in question has no other users

then its type is changed to dff.

If applicable, please suggest to reviewers how they can test the change.

Testcase included.