Checks for `assign` statements in netlists is missing from OpenLane 2

[donn]

Description

Some input netlists from non-Yosys tools may emit assign statements, which just tend to wreak havoc on some open-source tools (we think it's triggering a bug of some kind in OpenROAD.)

OpenLane 1 has a check for this, but this check does not exist in OpenLane 2.

Proposal

OpenLane 2 should have a similar check.