LifetimeDependenceDiagnostics: handle dynamic casting of Span<T>
Add a special case for checked_cast_addr_br instruction. If it conformed to
SourceDestAddrInstruction, then the diagnostics would already have handled it
naturally, but the instruction's conditional semantics are strange enough that
such a conformance might confuse other passes.
rdar://159793739 (Using `as?` with non-escapable types emits faulty lifetime
diagnostics)
