[RewriteStatepointsForGC] Bugfix for change 246133

Fix a bug in change 246133. I didn't handle the case where we had a cycle in the use graph and could add an instruction we were about to erase back on to the worklist. Oddly, I have not been able to write a small test case for this, even with the AssertingVH added. I have confirmed the basic theory for the fix on a large failing example, but all attempts to reduce that to something appropriate for a test case have failed.

Differential Revision: http://reviews.llvm.org/D12575

llvm-svn: 246718