This tests an issue related to Vivado 2017.2 vs 2017.3 changing MUXF8 behavior
The general issue is the LUT6_2 cannot be used with a MUXF8 (even if O5 is unused)
-2017.2: LUT6_2 works with MUXF8
-2017.3: LUT6_2 does not work with MUXF8
-All: LUT6 works with MUXF8
-All: MUXF8 (even with MUXF7) can be instantiated unconnected
-2017.4 seems to behave like 2017.3