[WIP] Demo a functional problem from D81647 with a fix and test case

[reames]

Summary

When spilling gc operands, we have to mark them as load and store. They were originally defs and we need to preserve that.

This is semantically important as a relocating collector *will* update them sometimes.

[dantrushin]

I will incorporate this into D81647, since fix will be even simpler there.

llvm/lib/CodeGen/FixupStatepointCallerSaved.cpp

238

Defs are never seen here by construction of OpsToSpill

248

Can iterate over MI.defs() (not operands()) and delete these checks