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

8223137: Rename predicate 'do_unroll_only()' to 'is_unroll_only()'. Reviewed-by: vlivanov