Add functions 'hasPredecessor' and 'hasPredecessorHelper' to SDNode. The

hasPredecessorHelper function allows predecessors to be cached to speed up
repeated invocations. This fixes PR10186.

X.isPredecessorOf(Y) now just calls Y.hasPredecessor(X)

Y.hasPredecessor(X) calls Y.hasPredecessorHelper(X, Visited, Worklist) with
empty Visited and Worklist sets (i.e. no caching over invocations).

Y.hasPredecessorHelper(X, Visited, Worklist) caches search state in Visited
and Worklist to speed up repeated calls. The Visited set is searched for X
before going to the worklist to further search the DAG if necessary.

llvm-svn: 134592