Reland of PPC/s390: SmiUntag only for 32bit

src/builtins/s390/builtins-s390.cc

Index: src/builtins/s390/builtins-s390.cc
diff --git a/src/builtins/s390/builtins-s390.cc b/src/builtins/s390/builtins-s390.cc
index 7c0a9ece07b9df63dba0820c5df9ceaaea5834a4..5311fcdfaa88432ba1e76aa205860f04f7aba63a 100644
--- a/src/builtins/s390/builtins-s390.cc
+++ b/src/builtins/s390/builtins-s390.cc
@@ -2214,11 +2214,16 @@ void Builtins::Generate_CallForwardVarargs(MacroAssembler* masm,
   {
     // Load the length from the ArgumentsAdaptorFrame.
     __ LoadP(r2, MemOperand(r5, ArgumentsAdaptorFrameConstants::kLengthOffset));
+#if V8_TARGET_ARCH_S390X
+    __ SmiUntag(r2);
+#endif
   }
   __ bind(&arguments_done);
 
   Label stack_empty, stack_done, stack_overflow;
+#if !V8_TARGET_ARCH_S390X
   __ SmiUntag(r2);
+#endif
   __ SubP(r2, r2, r4);
   __ CmpP(r2, Operand::Zero());
   __ ble(&stack_empty);