[unroll] Don't use a map from pointer to bool. Use a set.

[unroll] Don't use a map from pointer to bool. Use a set.

This is much more efficient. In particular, the query with the user
instruction has to insert a false for every missing instruction into the
set. This is just a cleanup a long the way to fixing the underlying
algorithm problems here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228994 91177308-0d34-0410-b5e6-96231b3b80d8