[turbofan] Treat uninitialized source positions as unknown.

src/compiler/graph-visualizer.cc

Index: src/compiler/graph-visualizer.cc
diff --git a/src/compiler/graph-visualizer.cc b/src/compiler/graph-visualizer.cc
index 8fb14653e1997fbbdb82d566df78e728e10acc30..f0e828f98e01f1be45d9288eeeb806ee8a1b560a 100644
--- a/src/compiler/graph-visualizer.cc
+++ b/src/compiler/graph-visualizer.cc
@@ -122,8 +122,7 @@ class JSONGraphNodeWriter {
       os_ << ",\"rankInputs\":[0]";
     }
     SourcePosition position = positions_->GetSourcePosition(node);
-    if (!position.IsUnknown()) {
-      DCHECK(!position.IsInvalid());
+    if (position.IsKnown()) {
       os_ << ",\"pos\":" << position.raw();
     }
     os_ << ",\"opcode\":\"" << IrOpcode::Mnemonic(node->opcode()) << "\"";
@@ -650,8 +649,7 @@ void GraphC1Visualizer::PrintSchedule(const char* phase,
         }
         if (positions != NULL) {
           SourcePosition position = positions->GetSourcePosition(node);
-          if (!position.IsUnknown()) {
-            DCHECK(!position.IsInvalid());
+          if (position.IsKnown()) {
             os_ << " pos:" << position.raw();
           }
         }