[turbofan] fix vreg mapping for instruction selector tests

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index 5811d548ff46bfaefe05d67d6c3f83abd1bb87aa..0fe0b0e380b0923ea5053aba12fc8380447af558 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -828,6 +828,8 @@ class InstructionSequence FINAL {
   BasicBlock* GetBasicBlock(int instruction_index);
 
   int GetVirtualRegister(const Node* node);
+  // TODO(dcarney): find a way to remove this.
+  const int* GetNodeMapForTesting() const { return node_map_; }
 
   bool IsReference(int virtual_register) const;
   bool IsDouble(int virtual_register) const;