[10] RFR(M): 8176506: C2: loop unswitching and unsafe accesses cause crash

[Roland Westrelin]

> But the real goal of that ByteBuffer test (and indeed all of the
> ByteBuffer optimizations) is to provide access to raw memory "as fast
> as C", and has implications for project Valhalla, etc.

In the case of the ByteBuffer test, with the changes I'm proposing there
will be an extra null check to verify accesses are on heap. That null
check will be out of loop so essentially free and there won't be any
regression. Off heap accesses or a mix of on heap and off heap accesses
in the same method would regress.

Roland.