Treat length of sequences as a "slot" in Useless optimization

[MatthewFluet]

In the Value.t representation of the Useless optimization, sequence and tuple elements are "slots", which combine a Useful.t lattice element with an Exists.t lattice element. When a value flows, vectors and tuples coerce the Useful.t component of slots but unify the Exists.t component. This ensures that agree on whether or not the element exists even if they disagree that the element is useful. If the "from"'s element is useful (and necessarily existing) but the "to"'s element is not useful, then forcing the "to"'s element to exist avoids a potentially expensive runtime coercion (e.g., to drop a component of a tuple or, worse, to drop a component of a tuple that is the contents of a vector).

Previously, the length of a sequence was represented simply as a Useful.t; this allowed a vector (whose contents were not useful) with a useful length to flow to a vector (whose contents are necessarily not useful) with a useless length. 1aad5fe (Optimize representation of sequences in Useless pass; #569) allowed the Useless optimization to change the representation of sequence with useless contents. In particular, a vector with useless contents but useful length becomes a word64 and a vector with useless contents and useless length becomes a unit.

However, when such vectors are themselves components of a tuple, then the program may have a flow of tuples, where the source tuple is changed from (..., ?? vector, ...) to (..., word64, ...) but the destination tuple is changed from (..., ?? vector, ...) to (..., unit, ...). (Note that the unification of the Exist.t components of the corresponding tuple elements is what makes the destination tuple have a unit element.)

This commit treats the length of arrays and vectors as a "slot", so that they track both usefulness and existence. Now, a vector with useless contents but a length that must exist becomes a word64 (even if the length is useless) and a vector with useless contents and a length that need not exist (and is necessarily useless) becomes a unit.

Fixes #585.

[fweimer]

Presumably a vector with useless contents but useless length should be a vector with useful contents but useless length.

[MatthewFluet]

No, the typo was the other way around. It should read a vector with useless contents but useful length becomes a word64. I'll fix the commit message and the PR text.