[FIRRTL] IMDCE: not removing all dead instances

[youngar]

For the following test, I expect both instances m and p to be removed, but for some reason only p is removed:

circuit Foo:
  module Foo:
    input in : UInt<1>
    output out : UInt<1>
    output out2 : UInt<1>

    inst m of Child
    m.in <= in

    inst m2 of Child
    m2.in <= in
    out <= m2.out

    inst p of PassThrough
    p.in <= in

    inst p2 of PassThrough
    p2.in <= in
    out2 <= p2.out

  module PassThrough :
    input in : UInt<1>
    output out : UInt<1>
    out <= in

  module Child :
    input in : UInt<1>
    output out : UInt<1>
    inst output_chain of PassThrough
    output_chain.in <= in
    out <= output_chain.out

running with firtool imdce.fir gives:

// Generated by CIRCT firtool-1.42.0-60-g3093438b0
module Foo(
  input  in,
  output out,
         out2
);

  Child m (
    .in  (in),
    .out (/* unused */)
  );
  Child m2 (
    .in  (in),
    .out (out)
  );
  PassThrough p2 (
    .in  (in),
    .out (out2)
  );
endmodule

module PassThrough(
  input  in,
  output out
);

  assign out = in;
endmodule

module Child(
  input  in,
  output out
);

  PassThrough output_chain (
    .in  (in),
    .out (out)
  );
endmodule

This is a regression which was introduced in #5226.