[Analyzer] Crash fix for FindLastStoreBRVisitor

FindLastStoreBRVisitor tries to find the first node in the exploded graph where
the current value was assigned to a region. This node is called the "store
site". It is identified by a pair of Pred and Succ nodes where Succ already has
the binding for the value while Pred does not have it. However the visitor
mistakenly identifies a node pair as the store site where the value is a
`LazyCompoundVal` and `Pred` does not have a store yet but `Succ` has it. In
this case the `LazyCompoundVal` is different in the `Pred` node because it also
contains the store which is different in the two nodes. This error may lead to
crashes (a declaration is cast to a parameter declaration without check) or
misleading bug path notes.

In this patch we fix this problem by checking for unequal `LazyCompoundVals`: if
their region is equal, and their store is the same as the store of their nodes
we consider them as equal when looking for the "store site". This is an
approximation because we do not check for differences of the subvalues
(structure members or array elements) in the stores.

Differential Revision: https://reviews.llvm.org/D58067

llvm-svn: 353943