Compute, update and handle borrowed-from instruction in various utilities and passes.
Also, used borrowed-from to simplify `gatherBorrowIntroducers` and `gatherEnclosingValues`.
Replace those utilities by `Value.getBorrowIntroducers` and `Value.getEnclosingValues`, which return a lazily computed Sequence of borrowed/enclosing values.
Introduce a new instruction `dealloc_stack_ref ` and remove the `stack` flag from `dealloc_ref`.
The `dealloc_ref [stack]` was confusing, because all it does is to mark the deallocation of the stack space for a stack promoted object.
In OSSA, there is no reason for the cond_br instruction to take arguments.
In OSSA utilities, we have two types of block arguments: terminator
results and phis. cond_br arguments don't fit into either one of these
categories. Supporting them would be prohibitively complex. In fact, I
have no idea model it.
I have already made the assumption in several places that we don't
support cond_br args. It would be impossible to test all the code that
makes this assumption, so even if we wanted to support cond_br args,
they would always be broken in practice.
Simply mark them illegal in the SILVerifier and make sure no OSSA
passes generate them.
