[JumpThreading] Fix exponential time algorithm computing known values.

ComputeValueKnownInPredecessors has a "visited" set to prevent infinite
loops, since a value can be visited more than once.  However, the
implementation didn't prevent the algorithm from taking exponential
time. Instead of removing elements from the RecursionSet one at a time,
we should keep around the whole set until
ComputeValueKnownInPredecessors finishes, then discard it.

The testcase is synthetic because I was having trouble effectively
reducing the original.  But it's basically the same idea.

Instead of failing, we could theoretically cache the result instead.
But I don't think it would help substantially in practice.

Differential Revision: https://reviews.llvm.org/D54239

llvm-svn: 346562