Simplify some code related to x64 calling convention.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index c193bb30fdd935a4e810e378128d7a2007f94589..b38feb849e483a4cbea0a037771e7a31fd79c407 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -817,11 +817,7 @@ void MacroAssembler::CallApiFunctionAndReturn(Address function_address,
   bind(&delete_allocated_handles);
   movq(Operand(base_reg, kLimitOffset), prev_limit_reg);
   movq(prev_limit_reg, rax);
-#ifdef _WIN64
-  LoadAddress(rcx, ExternalReference::isolate_address(isolate()));
-#else
-  LoadAddress(rdi, ExternalReference::isolate_address(isolate()));
-#endif
+  LoadAddress(arg_reg_1, ExternalReference::isolate_address(isolate()));
   LoadAddress(rax,
               ExternalReference::delete_handle_scope_extensions(isolate()));
   call(rax);