JIT: tail merge suspension points

[jakobbotsch]

For GDVs it should be a common scenario that we have a join right after two separate runtime async calls assigning the same value. We should be able to (manually) tail merge the suspension points created for these to remove one of them entirely.

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[jakobbotsch]

It would be interesting to look into whether there are NativeAOT opportunities too, in which case this might be a nice size saving opportunity and we might want to prioritize it more.