Don't use std::set_difference when the two sets are sorted differently.  Compute the difference manually instead.
Don't use std::set_difference when the two sets are sorted differently.  Compute
the difference manually instead.

This allows GVNPRE to produce correct analysis for the example in the GVNPRE
paper.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@37425 91177308-0d34-0410-b5e6-96231b3b80d8

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