[analyzer] Don't create new PostStmt nodes if we don't have to.

Doing this caused us to mistakenly think we'd seen a particular state before
when we actually hadn't, which resulted in false negatives. Credit to
Rafael Auler for discovering this issue!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211209 91177308-0d34-0410-b5e6-96231b3b80d8