Switch -Wuninitialized to use a reverse-post order traversal as

Switch -Wuninitialized to use a reverse-post order traversal as
an initial baseline for enqueued blocks, but use a simple DFS stack
for propagating changes quickly up back edges.

This provides a 3.5% reduction in -fsyntax-only time on sqlite3.c.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168241 91177308-0d34-0410-b5e6-96231b3b80d8