DivergenceAnalysis: added debug output

Summary:
This commit does two things:

1. modified the existing DivergenceAnalysis::dump() so it dumps the
   whole function with added DIVERGENT: annotations;

2. added code to do that dump if the appropriate -debug-only option is
   on.

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47700

Change-Id: Id97b605aab1fc6f5a11a20c58a99bbe8c565bf83
llvm-svn: 336998