More cleanups with using the liveness analysis to removed dead symbols and

More cleanups with using the liveness analysis to removed dead symbols and
variables from the symbolic state. Now we keep a mapping from the predecessor
node of a statement and its cleaned state, and interpose an accessor
"GetState()" to return the cleaned state when the node is the predecessor node,
and the real state otherwise. This obviates problem of ever accidentally
cleaning the state more than once (thus blowing away new bindings by mistake).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@48124 91177308-0d34-0410-b5e6-96231b3b80d8