[FIRRTL] Instance cycle - diagnose, don't crash

[dtzSiFive]

Example:

FIRRTL version 4.0.0

circuit Cyclic:
  public module Cyclic:
    inst f of Foo

  module Foo:
    inst c of Cyclic

Infer Resets presently overflows its stack recursing. Skipping that, we produce:

// Generated by CIRCT firtool-1.76.0-22-g0b5534b31
module Cyclic();
  Foo f ();
endmodule

module Foo();
  Cyclic c ();
endmodule

Producing a diagnostic would be a good improvement.