splitcells - split up multi-bit cells

yosys> help splitcells
splitcells [options] [selection]
This command splits multi-bit cells into smaller chunks, based on usage of the
cell output bits.

This command operates only in cells such as $or, $and, and $mux, that are easily
cut into bit-slices.
-format char1[char2[char3]]
the first char is inserted between the cell name and the bit index, the
second char is appended to the cell name. e.g. -format () creates cell
names like 'mycell(42)'. the 3rd character is the range separation
character when creating multi-bit cells. the default is '[]:'.