Traversing an array of value types seen as an array of Object is slow

[Roland Westrelin]

> It seems that the JIT never try to optimize the loop for whatever
> reason so the loop is done by the interpreter and only the access to
> the array cell is JITed.

We currently don't support loading from an Object[] array that's
actually a flattened value type array because we can't tell at compile
time what value type is being loaded. That forces compiled code to
deoptimize.

Roland.