Make stack_parameter_count a plain register.

src/x64/deoptimizer-x64.cc

Index: src/x64/deoptimizer-x64.cc
diff --git a/src/x64/deoptimizer-x64.cc b/src/x64/deoptimizer-x64.cc
index a5e4583aa24b1fe2a9d9c5b9083f239a37cfaa86..bf11e0860f4b36e14be8ef340e17bd3145628029 100644
--- a/src/x64/deoptimizer-x64.cc
+++ b/src/x64/deoptimizer-x64.cc
@@ -106,10 +106,7 @@ void Deoptimizer::SetPlatformCompiledStubRegisters(
     FrameDescription* output_frame, CodeStubInterfaceDescriptor* descriptor) {
   intptr_t handler =
       reinterpret_cast<intptr_t>(descriptor->deoptimization_handler_);
-  int params = descriptor->register_param_count_;
-  if (descriptor->stack_parameter_count_ != NULL) {
-    params++;
-  }
+  int params = descriptor->environment_length();
   output_frame->SetRegister(rax.code(), params);
   output_frame->SetRegister(rbx.code(), handler);
 }