Unhandled node for delay estimation: smod

The following IR results in "Unhandled node for delay estimation in delay model 'sky130': **smod**" at code gen:

top fn test(i: bits[32]) -> bits[32] {
literal.2: bits[32] = literal(value=2, id=2, pos=[(1,2,5)])
smod.3: bits[32] = **smod**(i, literal.2, id=3, pos=[(1,2,5)])
literal.4: bits[32] = literal(value=3, id=4, pos=[(1,3,5)])
ret add.5: bits[32] = add(smod.3, literal.4, id=5, pos=[(1,3,5)])
}

Suggest adding, refining delay model for smod (and any other non-covered ir operants for successful code generation).

Example:
[test.ir.txt](https://github.com/google/xls/files/10884663/test.ir.txt)
[runme.sh.txt](https://github.com/google/xls/files/10884665/runme.sh.txt)