Request for reviews (XS): 6746907: Improve implicit null check generation

[Vladimir Kozlov]

http://webrev.invokedynamic.info/kvn/6746907/index.html

Fixed 6746907: Improve implicit null check generation

Problem:
VM doesn't generate implicit null check if there are empty blocks
between null test and uncommon_trap call.
Also VM specifies the holder of a tested oop in uncommon_trap's
debug info instead of passing NULL.
This increase registers pressure.

Solution:
Skip over empty blocks when looking for uncommon_trap
and add the missing offset check for compressed oops when
looking for an implicit null check opportunities.
Set tested oop's value to NULL in uncommon_trap's debug info.

Reviewed by:
Fix verified (y/n): y, generated code.

Other testing:
JPRT