Adjust enablement of RewriteStatepointsForGC

When back merging the changes in 229945 I noticed that I forgot to mark the test cases with the appropriate GC.  We want the rewriting to be off by default (even when manually added to the pass order), not on-by default.  To keep the current test working, mark them as using the statepoint-example GC and whitelist that GC.  

Longer term, we need a better selection mechanism here for both actual usage and testing.  As I migrate more tests to the in tree version of this pass, I will probably need to update the enable/disable logic as well. 

llvm-svn: 229954