LRB midpath code quality

[Roman Kennke]

>> Well yes, an implicit null-check would be good. It would never fire,
>> because if the value is null, it would blow up earlier in the cset
>> check. Or am I missing something?
> 
> I suppose PhaseCFG::implicit_null_check() doesn't recognize the barrier
> load as a candidate for implicit null checks for some reason.

Uhhh. I think I may have removed the -8 handling from 
MacroAssembler::needs_explicit_null_check() path in original LRB patch :-)

Let me try to put that back...

Roman