S390: Fix Register conflict in CEntryStub

when needs_return_buffer == true and result_size() > 2, the 3rd return value
is placed in r4 register. But when debug code is emited, it uses r4 register
for checking exceptions, which corrupts the 3rd return value.

R=joransiu@ca.ibm.com, mbrandy@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1839663002

Cr-Commit-Position: refs/heads/master@{#35084}

src/s390/code-stubs-s390.cc

@@ -1104,9 +1104,9 @@ void CEntryStub::Generate(MacroAssembler* masm) {
     Label okay;
     ExternalReference pending_exception_address(
         Isolate::kPendingExceptionAddress, isolate());
-    __ mov(r4, Operand(pending_exception_address));
-    __ LoadP(r4, MemOperand(r4));
-    __ CompareRoot(r4, Heap::kTheHoleValueRootIndex);
+    __ mov(r1, Operand(pending_exception_address));
+    __ LoadP(r1, MemOperand(r1));
+    __ CompareRoot(r1, Heap::kTheHoleValueRootIndex);
     // Cannot use check here as it attempts to generate call into runtime.
     __ beq(&okay, Label::kNear);
     __ stop("Unexpected pending exception");