The LocalStackSlotPass pre-allocates a stack protector and makes sure that it comes before the local variables on the stack.
We need to make sure that later during PEI we don't re-allocate a new stack protector slot. If that happens, the new stack protector slot will end up being after the local variables that it should be protecting.
Therefore, we would have two slots assigned for two different stack protectors, one at the top of the stack, and one at the bottom. Since PEI will overwrite the assigned slot for the stack protector, the load that is used to compare the value of the stack protector will use the slot assigned by PEI, which is wrong.
For this, we need to check if the object is pre-allocated, and re-use that pre-allocated slot.