The optimizer may completely remove all ``memoryguard``s preventing stack-to-memory.

Came up in https://github.com/ethereum/solidity/pull/12762.

We made ``memoryguard`` movable and removable to allow it to be CSE'd on the one hand and not to clutter the yul code with ``pop(memoryguard(...))`` on the other hand (also movable implies removable at the moment, so movable, but not *re*movable doesn't work).

Previously, the ``memoryguard``s would still never be removed entirely, since we'd always have a ``mstore(0x40, memoryguard(...))``, which depends on the actual value and could not be resolved.
Ever since the unused store eliminator, this ``mstore`` may be eliminated, though, which can result in the ``memoryguard`` to vanish, even though such code is succinctly memory-safe (these are cases that don't use any dynamic memory at all after optimization!).

With https://github.com/ethereum/solidity/pull/12762 such situations would become even more likely.

So we need a workaround for this. I have several options in mind, but none of them are overly nice. We can try to prevent the optimizer from removing the last ``memoryguard`` (hacky)... we could check for the memoryguard at the beginning of running the optimizer and then check again, if it has vanished and add it back as plain ``pop(memoryguard(...))`` (may be fine, but also a bit weird and kind of against the idea of ``memoryguard``s)... we could say that a vanishing ``memoryguard`` means no dynamic memory use and have the ``StackToMemoryMover`` still run and *introduce* a new ``memoryguard`` in these cases... not sure which is the least hacky option...

Start by reading the optimizer behavior described here and the context in PR #12762, then trace the unused store eliminator and StackToMemoryMover around memoryguard handling. Compare the proposed workarounds and add regression coverage for optimized code where all memoryguards disappear; done means memory-safe code still receives the required stack-to-memory treatment.

Written by the indexing model from the issue text.