message: Add comment to explain the change to I64ExtendI32U.
diff: @@ -4052,6 +4052,9 @@ impl FunctionCodeGenerator<CodegenError> for X64FunctionCode {
  loc,
  ret,
  );
  +
  + // A 32-bit memory write does not automatically clear the upper 32 bits of a 64-bit word.
  + // So, we need to explicitly write zero to the upper half here.
  if let Location::Memory(base, off) = ret {
  Self::emit_relaxed_binop(
  a,
  