Fix #196. Errorneus tail-optimization of inner DefDef's

Unless a DefDef is a result of desugaring an exception handler it shouldn't be optimized by tailrec.

There's a potential improvement here: to detect that a DefDef
 is a label def which itself is called only in tail position.

 This is a requirement to implement #194
Merge pull request #197 from dotty-staging/fix/tailrec-defdef

Fix #196. Errorneus tail-optimization of inner DefDef's