Fix Dfg circular driver tracing with partial assignments

src/V3DfgBreakCycles.cpp

@@ -187,7 +187,8 @@ class TraceDriver final : public DfgVisitor {
             // as a splice cannot be fed into arbitray logic
             if (DfgVertexSplice* const splicep = resp->cast<DfgVertexSplice>()) {
                 DfgVertexVar* const tmpp = createTmp("TraceDriver", splicep);
-                splicep->replaceWith(tmpp);
+                // Note: we can't do 'splicep->replaceWith(tmpp)', as other
+                // variable sinks of the splice might have a defaultp driver.
                 tmpp->srcp(splicep);
                 resp = tmpp;
             }

test_regress/t/t_dfg_break_cycles.v

@@ -249,4 +249,16 @@ module t (
   `signal(ARRAY_4, 32);
   assign ARRAY_4  = array_4[2];
 
+  logic [1:0] packed_0; // UNOPTFLAT
+  logic       packed_0_lsb;
+  always_comb begin
+   packed_0[1] = rand_b[1];
+   packed_0_lsb = packed_0[0];
+  end
+  always_comb packed_0[0] = rand_b[0];
+  assign PACKED_0 = packed_0;
+  `signal(PACKED_0, 2);
+  `signal(PACKED_0_LSB, 1);
+  assign PACKED_0_LSB = packed_0_lsb;
+
 endmodule