[elements] Improve Array.prototype.splice speed

By using memmove for SMI elements we get a roughly 3x speedup over the slower
iterative copying with write barriers.

Bug: chromium:835558
Change-Id: I73da07a1648a3495ff78212ffa1ed949d205a7d2
Reviewed-on: https://chromium-review.googlesource.com/1028236Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52792}

src/elements.cc

@@ -2298,6 +2298,9 @@ class FastElementsAccessor : public ElementsAccessorBase<Subclass, KindTraits> {
       if (IsDoubleElementsKind(KindTraits::Kind)) {
         MemMove(dst_elms->data_start() + dst_index,
                 dst_elms->data_start() + src_index, len * kDoubleSize);
+      } else if (IsSmiElementsKind(KindTraits::Kind)) {
+        MemMove(dst_elms->data_start() + dst_index,
+                dst_elms->data_start() + src_index, len * kPointerSize);
       } else {
         DisallowHeapAllocation no_gc;
         heap->MoveElements(FixedArray::cast(*dst_elms), dst_index, src_index,