[turbofan] Treat uninitialized source positions as unknown.

src/compiler/source-position.cc

Index: src/compiler/source-position.cc
diff --git a/src/compiler/source-position.cc b/src/compiler/source-position.cc
index 97577c7966692e281fd0d0d57c60d25767ed5887..93f89e03bc0c20c541b8537302fd387ac57904e5 100644
--- a/src/compiler/source-position.cc
+++ b/src/compiler/source-position.cc
@@ -16,7 +16,6 @@ class SourcePositionTable::Decorator final : public GraphDecorator {
       : source_positions_(source_positions) {}
 
   void Decorate(Node* node, bool incomplete) final {
-    DCHECK(!source_positions_->current_position_.IsInvalid());
     source_positions_->table_.Set(node, source_positions_->current_position_);
   }
 
@@ -28,7 +27,7 @@ class SourcePositionTable::Decorator final : public GraphDecorator {
 SourcePositionTable::SourcePositionTable(Graph* graph)
     : graph_(graph),
       decorator_(nullptr),
-      current_position_(SourcePosition::Invalid()),
+      current_position_(SourcePosition::Unknown()),
       table_(graph->zone()) {}
 
 
@@ -56,7 +55,7 @@ void SourcePositionTable::Print(std::ostream& os) const {
   bool needs_comma = false;
   for (auto i : table_) {
     SourcePosition pos = i.second;
-    if (!pos.IsUnknown()) {
+    if (pos.IsKnown()) {
       if (needs_comma) {
         os << ",";
       }