Review request (XS): 6684385 Loop unswitching crashes without LoopNode

[Chuck Rasbold]

http://webrev.invokedynamic.info/rasbold/6684385/

Fixed 6684385: Loop unswitching crashes without LoopNode

Due to the sometimes unpredictable handling of irreducible loops, a
non-irreducible loop was found on the second call to
build_loop_tree(), and therefore was not beautified.

Without a LoopNode at the head of the loop, policy_unswitching()
crashes.

Fix policy_unswitching() to return false for any loop which does not
have a LoopNode for a head. Do the same for partial_peel().

Reviewed by:
Fix verified (y/n): y