SSA: Fix cycle in stack shuffler

Previously, when we were trying to detect if all slots are reachable or
final, we were checking only target slots that are filled at the current
moment.
However, we need to check *all* the target slots, even those beoynd the
current stack size.

Previous behaviour meant we were not shrinking the stack as much as was
required in some situations and the shuffling went into a loop.

With this fix most of the examples that were looping previously are now
successful.