8223138: Small clean-up in loop-tree support. Summary: Rename predicate 'is_inner()' to 'is_innermost()' to be accurate. Added 'is_root()' predicate for root parent test in loop-tree. Changed definition of 'is_loop()' to always lazy-read the tail, since it should never be NULL. Cleanup of 'tail()' definition. Reviewed-by: vlivanov, neliasso

/build/
/dist/
/.idea/
nbproject/private/
/webrev
/.src-rev
/.jib/
.DS_Store
.metadata/
.recommenders/
test/nashorn/script/external
test/nashorn/lib
NashornProfile.txt
**/JTreport/**
**/JTwork/**