[silgen] Add a new isa<...> API for ManagedValue.
[silgen] Add a new isa<...> API for ManagedValue.

This lets you do isa<AllocRefInst> to determine if the underlying ManagedValue
contains within it an AllocRefInst without needing to call getValue(). I would
like to restrict the number of places where getValue() needs to be called as
much as possible.

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