[SystemZ] lowerDYNAMIC_STACKALLOC_XPLINK - use cast<> instead of dyn_cast<> to...

[SystemZ] lowerDYNAMIC_STACKALLOC_XPLINK - use cast<> instead of dyn_cast<> to avoid dereference of nullptr

The pointer is always dereferenced, so assert the cast is correct instead of returning nullptr