Make stack_parameter_count a plain register.

src/deoptimizer.cc

Index: src/deoptimizer.cc
diff --git a/src/deoptimizer.cc b/src/deoptimizer.cc
index 819b5e94a30a1753ee4c035cb81724e2c8d6bbee..2fc04fa6e051bce5609f453f23944ed7439326b1 100644
--- a/src/deoptimizer.cc
+++ b/src/deoptimizer.cc
@@ -1494,7 +1494,7 @@ void Deoptimizer::DoComputeCompiledStubFrame(TranslationIterator* iterator,
   }
 
   intptr_t caller_arg_count = 0;
-  bool arg_count_known = descriptor->stack_parameter_count_ == NULL;
+  bool arg_count_known = !descriptor->stack_parameter_count_.is_valid();
 
   // Build the Arguments object for the caller's parameters and a pointer to it.
   output_frame_offset -= kPointerSize;