Adjust how we avoid poll insertion inside the poll function (NFC)

I realized that my early fix for this was overly complicated.  Rather than scatter checks around in a bunch of places, just exit early when we visit the poll function itself.

Thinking about it a bit, the whole inlining mechanism used with gc.safepoint_poll could probably be cleaned up a bit.  Originally, poll insertion was fused with gc relocation rewriting.  It might be worth going back to see if we can simplify the chain of events now that these two are seperated.  As one thought, maybe it makes sense to rewrite calls inside the helper function before inlining it to the many callers.  This would require us to visit the poll function before any other functions though..

llvm-svn: 228634