Code review request: 7151580 - Separate DA/DU logic from exception checking logic in Flow.java

[Maurizio Cimadamore]

Hi,
this is a biggie changeset [1] for separating the definite assignment 
logic from the exception analysis logic in Flow.java. The two logic used 
to share the same visitor - moving forward it would be nice to split the 
two routines in two separate visitors as there are use cases in which we 
might only need to run one of the two (i.e. when inferring thrown types 
of a lambda).

The inspiration for this work came from the BGGA prototype which 
featured a similar refactoring. I have added my own little twist to the 
code - for example, both visitors are nested classes inside Flow - they 
also share a common super class that contains shared routine for 
handling jumps.

[1] - http://cr.openjdk.java.net/~mcimadamore/7151580.0/webrev/ 
<http://cr.openjdk.java.net/%7Emcimadamore/7151580.0/webrev/>

Maurizio