prjxray/fuzzers/012-clbn5ffmux

Purpose:
Document A5FFMUX family of CLB muxes

Algorithm:
*5FFMUX
Inputs can come from either the LUT6_2 NO5 output or the CLB NX input
To perturb the CLB the smallest, want LUT6 always instantiated
However, some routing congestion that would require putting FFs in bypass
(which turns out is actually okay, but didn't realize that at the time)
Decided instead ot instantiate LUT8, but not use the output
Turns out this is okay and won't optimize things away
So then, the 5FF D input is switched between the O5 output and an external CLB input

Outcome:
Bits are one hot encoded per mux position