Currently, stack protector loads and stores are resolved during LocalStackSlotAllocation (if the pass needs to run). When this is the case, the base register assigned to the frame access is going to be one of the vregs created during LocalStackSlotAllocation. This means that we are keeping a pointer to the stack protector slot, and we're using this pointer to load and store to it.
In case register pressure goes up, we may end up spilling this pointer to the stack, which can be a security concern.
Instead, leave it to PEI to resolve the frame accesses. In order to do that, we make all stack protector accesses go through frame index operands, then PEI will resolve this using an offset from sp/fp/bp.