muxpack - $mux/$pmux cascades to $pmux

yosys> help muxpack
muxpack [selection]
This pass converts cascaded chains of $pmux cells (e.g. those create from case
constructs) and $mux cells (e.g. those created by if-else constructs) into
$pmux cells.

This optimisation is conservative --- it will only pack $mux or $pmux cells
whose select lines are driven by '$eq' cells with other such cells if it can be
certain that their select inputs are mutually exclusive.