[turbofan] Treat uninitialized source positions as unknown.

src/compiler/instruction.cc

Index: src/compiler/instruction.cc
diff --git a/src/compiler/instruction.cc b/src/compiler/instruction.cc
index cea74a70123f2db534ee23d3b93f42e783a808bc..3faac595a19c502c57f6ee30ed2664ccb7d096ef 100644
--- a/src/compiler/instruction.cc
+++ b/src/compiler/instruction.cc
@@ -657,8 +657,6 @@ bool InstructionSequence::GetSourcePosition(const Instruction* instr,
 
 void InstructionSequence::SetSourcePosition(const Instruction* instr,
                                             SourcePosition value) {
-  DCHECK(!value.IsInvalid());
-  DCHECK(!value.IsUnknown());
   source_positions_.insert(std::make_pair(instr, value));
 }