s390:[wasm-simd] fix unpack low/high on the simulator

Change-Id: I1c7715d5133bc9fb0711c8d9922e2ca31ed37042
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2426947Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#70100}

src/execution/s390/simulator-s390.cc

@@ -3407,7 +3407,7 @@ template <class S, class D>
 void VectorUnpackHigh(void* dst, void* src) {
   D value = 0;
   for (size_t i = 0; i < kSimd128Size / sizeof(D); i++) {
-    value = *(reinterpret_cast<S*>(src) + i + (sizeof(S) / 2));
+    value = *(reinterpret_cast<S*>(src) + i);
     memcpy(reinterpret_cast<D*>(dst) + i, &value, sizeof(D));
   }
 }
@@ -3463,9 +3463,10 @@ EVALUATE(VUPLH) {
 template <class S, class D>
 void VectorUnpackLow(void* dst, void* src) {
   D value = 0;
-  for (size_t i = kSimd128Size / sizeof(D); i > 0; i--) {
-    value = *(reinterpret_cast<S*>(src) + i - 1);
-    memcpy(reinterpret_cast<D*>(dst) + i - 1, &value, sizeof(D));
+  size_t count = kSimd128Size / sizeof(D);
+  for (size_t i = 0; i < count; i++) {
+    value = *(reinterpret_cast<S*>(src) + i + count);
+    memcpy(reinterpret_cast<D*>(dst) + i, &value, sizeof(D));
   }
 }