S390: Fix the order of base, index register in codegen

MemOperand is declared as:
````
MemOperand(Register rx, Register rb, Disp offset = 0);
```

Change-Id: I56b89dec8f206081f08c674fc160b6bc2347877b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3001173Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#75530}

src/compiler/backend/s390/code-generator-s390.cc

@@ -131,7 +131,7 @@ class S390OperandConverter final : public InstructionOperandConverter {
 #if V8_TARGET_ARCH_S390X && !V8_TARGET_LITTLE_ENDIAN
     // We want to read the 32-bits directly from memory
     MemOperand mem = InputStackSlot(index);
-    return MemOperand(mem.rb(), mem.rx(), mem.offset() + 4);
+    return MemOperand(mem.rx(), mem.rb(), mem.offset() + 4);
 #else
     return InputStackSlot(index);
 #endif