dfflibmap violates selection through dfflegalize

[widlarizer]

Version

main

On which OS did this happen?

Linux

Reproduction Steps

read_verilog tests/sim/dffe.v; proc; techmap; stat; dfflibmap -liberty tests/techmap/dfflibmap.lib nothing; stat

Expected Behavior

nothing is an empty selection so the design should be unchanged

Actual Behavior

nothing is an empty selection and you get a warning for it, but dfflibmap calls calls dfflegalize on the whole design despite dfflegalize supporting selections too. This threw me off when investigating test failures, because I was doing

copy top top_unmapped
dfflibmap -liberty dfflibmap_dffn_dffe.lib top

and it would modify top_unmapped which I saved to prevent modifying it