RFR(S): barrier moved due to null checks needs to always fix memory edges

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/shenandoah/wbatnullcheckfixmem/webrev.00/

This is a crash Aleksey observed. When a null check dominates a write
barrier, we move the barrier up so the null check becomes an implicit
null check. When the write barrier is moved up, memory edges must be
tweaked. Current code doesn't fix memory edges if the memory input to
the write barrier doesn't change. That's incorrect because the barrier
might now dominate some uses that it didn't dominate before.

Roland.