Mark DPICallIntrinsicOps as over-defined in the FIRRTL IMConstProp pass to prevent connections to the call's result from being removed.

Running this Chisel 7 snippet through firtool

class DPITest extends Module {
  val out = IO(Output(Bool()))
  val bar = Wire(Bool())
  val foo = RawClockedNonVoidFunctionCall("foo", Bool())(this.clock, true.B, bar)
  bar := RawClockedNonVoidFunctionCall("bar", Bool())(this.clock, true.B, foo)
  out := bar
}

incorrectly produces

[...]
  always @(posedge clock) begin
    foo(1'bz, _foo_0); 
    bar(_GEN, _bar_0);  
    _GEN_0 <= _bar_0; 
  end // always @(posedge)
  assign out = 1'bz; 
endmodule

The DPI call intrinsics ale initialized as Unknown in the constant propagation lattice and due to their cyclic dependency, this never gets resolved. This eventually leads to the connection to the bar wire being removed IMConstProp.cpp:1091:

if (isDeletableWireOrRegOrNode(destOp) &&
                !isOverdefined(fieldRef)) {
              connect.erase();
              ++numErasedOp;
            }

Initializing the call results to Overdefined prevents this.

I'm not familiar with this pass, but the !isOverdefined(fieldRef) check before deleting a connection looks a bit too optimistic to me. Is it an invariant at this point that every value is known to be either constant or overdefined? If so, this should probably get an assert, right?