[9] RFR (XS): 8058847: C2: EliminateAutoBox regression after 8042786

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8058847/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8058847

LoadNode::eliminate_autobox() doesn't care whether the unboxing node 
represents signed or unsigned load. It means that for LoadUB/LoadUS -> 
... -> LoadB/LoadS unboxing/boxing pair, information about unsigned load 
is lost. It leads to incorrect code generation.

The fix is to reify this information in the graph by wrapping the result 
in AddI (_ mask).

Testing: regression test

Best regards,
Vladimir Ivanov