[RewriteStatepointsForGC] Use DenseSet in place of std::set [NFC]

This should be the last cleanup on non-llvm preferred data structures.  I left one use of std::set in an assertion; DenseSet didn't seem to have a tombstone for CallSite defined.  That might be worth fixing, but wasn't worth it for a debug only use.

llvm-svn: 230084