Eliminate O(n^2) worst-case behavior in SSA construction

Eliminate O(n^2) worst-case behavior in SSA construction

The code uses a priority queue and a worklist, which share the same
visited set, but the visited set is only updated when inserting into
the priority queue. Instead, switch to using separate visited sets
for the priority queue and worklist.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234425 91177308-0d34-0410-b5e6-96231b3b80d8