[AddressLowering] Shorten stack lifetimes.
During storage allocation, create all alloc_stacks at the front of the
function entry block, except those for opened existential types. Do not
create any dealloc_stacks, even for opened existential types.
Then, after function rewriting, position the alloc_stacks according to
uses, except those for opened existential types. This means allocating
in the block that's the least common ancestor of all uses. At this
point, create dealloc_stacks on the dominance frontier of the
alloc_stacks, even for allocations for opened existential types.
The behavior for opened existential types diverges from all others in
order to maintain SIL validity (as much as possible) while the pass
runs. It would be invalid to create the alloc_stacks for opened
existential types at the front of the entry block because the type which
is opened is not yet available, but that type's opening must dominate
the corresponding alloc_stack.
