[FIRRTL] CheckCombLoops: does not find all comb loops

[youngar]

FIRRTL version 4.0.0
circuit Foo:
  public module Foo:
  public module Bar:
    output o : UInt<8>
    wire w : UInt<8>
    connect w, w
    connect o, w

./bin/firtool ./comb-loops.fir gives:

// Generated by CIRCT circtorg-0.0.0-4546-g7df593d08-dirty
module Foo();
endmodule

module Bar(
 output [7:0] o
);

 assign o = 8'bz;
endmodule

We should have detected the combinational loop of wire w, and we should never allow a port to be driven with z. This is related to the fact that CheckCombLoops was never updated to handle more than one top level module.