- Add explicitly handling of A_WIDTH=1 for completeness
- mux2 uses one LUT3 instead of a hard mux (which did use LUTs anyway)
- mux4 uses one LUT4 instead of hard muxes (which did use LUTs anyway)
- mux8 uses only bottom half of a slice
- Add a mux12 for intermediate variant between mux8 and mux16
- For sizes larger than 16 inputs, instantiate the right mux size
- More comments about implementation choices
- More tests including with -widemux and -abc9, and more comments
The presence of IS_*_INVERTED on FD* cells follows Vivado, which
apparently has been decided by a dice roll. Just assume false if the
parameter doesn't exist.
Fixes#2559.
