[turbofan] First step towards correctified 64-bit addressing.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index cd11f1a6b75f1b1590f375b4f37ecb84c2147b89..8c0255e9abbb8dbb38c76507315edb7d0fb62703 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -1147,10 +1147,12 @@ Node* SimplifiedLowering::ComputeIndex(const ElementAccess& access,
     index = graph()->NewNode(machine()->Int32Add(), index,
                              jsgraph()->Int32Constant(fixed_offset));
   }
-  // TODO(bmeurer): 64-Bit
-  // if (machine()->Is64()) {
-  //   index = graph()->NewNode(machine()->ChangeInt32ToInt64(), index);
-  // }
+  if (machine()->Is64()) {
+    // TODO(turbofan): This is probably only correct for typed arrays, and only
+    // if the typed arrays are at most 2GiB in size, which happens to match
+    // exactly our current situation.
+    index = graph()->NewNode(machine()->ChangeUint32ToUint64(), index);
+  }
   return index;
 }