[GC docs] Add example IR, assembly, and stackmaps to Statepoint documentation

[GC docs] Add example IR, assembly, and stackmaps to Statepoint documentation

When I originally committed the statepoint docs, I left placeholders for example IR fragments.  I'm finally getting around to filling those in.

I also added IR fragments to illustrate the usage of the PlaceSafepoints pass while I was at it.

llvm-svn: 230601