MIPS: Fix bad RegisterConfiguration usage in InstructionSequence unit tests.

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index 5d8864eea7d006197a54c19d6e44b6f7cbf2dd62..cf60e18caeb6c6ca9367d8df155a1f76a2b46b1a 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -1505,6 +1505,8 @@ class V8_EXPORT_PRIVATE InstructionSequence final
   void ValidateDeferredBlockEntryPaths() const;
   void ValidateSSA() const;
 
+  const RegisterConfiguration* GetRegisterConfigurationForTesting();
+
  private:
   friend V8_EXPORT_PRIVATE std::ostream& operator<<(
       std::ostream& os, const PrintableInstructionSequence& code);