RFR(M): 8181211: C2: Use profiling data to optimize on/off heap unsafe accesses

[Roland Westrelin]

Hi Paul,

> A further clean up of the ByteBuffer code is to unify heap and direct
> implementations to consistently use the double addressing mode, where
> for the latter a null from a field is read and passed for the heap
> object. We conservatively held off doing that due to performance
> concerns. Do you think your patch will help here too?

It would help if the application either only uses on heap accesses or
off heap accesses. A mix of the two would cause profile pollution and
a performance drop. 

MethodHandles.byteBufferViewVarHandle() shares one implementation for
both off and on heap accesses, right? AFAICT, it is subject to that
problem. 

Roland.