[StaticAnalyzer] LoopUnrolling fixes

1. The LoopUnrolling feature needs the LoopExit included in the CFG so added this
dependency via the config options
2. The LoopExit element can be encountered even if we haven't encountered the 
block of the corresponding LoopStmt. So the asserts were not right.
3. If we are caching out the Node then we get a nullptr from generateNode which
case was not handled.


Differential Revision: https://reviews.llvm.org/D37103

llvm-svn: 311880