RFR(S): 8238723: yank_alloc_node must remove membar

[Vladimir Ivanov]

> Webrev: http://cr.openjdk.java.net/~neliasso/8238723/webrev.01/

Unfortunately, I don't fully understand how the fix works.

Considering possible usages of result projection from Allocate, 
Initialize is already removed by the time yank_alloc_node is called. So, 
we are left with MemBar and AddP cases.

In case of MemBar I assume you are handling the precedence edge 
(MemBarNode::Precedent). Why do you replace it with TOP? I assume you 
want to eliminate the barrier, but I don't see how it helps.

What are the cases when AddPs on raw oop are present? Is it safe to 
remove Allocate node in such cases?

Best regards,
Vladimir Ivanov

[1] 
http://hg.openjdk.java.net/jdk/jdk/file/tip/src/hotspot/share/opto/escape.cpp#l1912