[ty] Propagate deletions through nested loop headers (#28044)
An inner loop can delete a variable on one iteration and then exit
through `break` on a later iteration. The variable can remain unbound
after the inner loop and on subsequent outer iterations, but ty did not
propagate deletions carried by nested loop headers into the enclosing
loop's boundness analysis.
Include nested headers' deletion reachability when computing an
enclosing header's reachability. Keep the loop-back bindings intact for
type inference, and memoize the structural traversal used during Salsa
cycle initialization so shared descendant headers are only visited once.
Related to #28009.
## Test plan
- Add `while` and `for` mdtests for deletions that survive an inner-loop
iteration and a later `break`, including an explicit `continue` and a
`for`-`else` that returns.
- Verify that reassignment after the inner loop restores boundness on
subsequent outer iterations.
- Verify that statically unreachable deletions preserve boundness and
literal types through several nested loops.
