[analyzer] Fix a spurious undef value warning.

When we timeout or exceed a max number of blocks within an inlined
function, we retry with no inlining starting from a node right before
the CallEnter node. We assume the state of that node is the state of the
program before we start evaluating the call. However, the node pruning
removes this node as unimportant. 

Teach the node pruning to keep the predecessors of the call enter nodes.

llvm-svn: 157860