RFR: Disable over-strong assert

[Roman Kennke]

I recently put an assert in maybe_update_heap_oop() which asserts that
when a Java thread beat us with a store, that the stored value is
to-space. There's a small gotcha there: if the Java thread beat us with
an arraycopy, there is a small window where we can see from-space
values, before the Java thread calls the post-ref-array-barrier.

http://cr.openjdk.java.net/~rkennke/disable-assert/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/disable-assert/webrev.00/>

Roman