[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230601 91177308-0d34-0410-b5e6-96231b3b80d8