fixes and improves label handling in the IR theory

[ivg]

For the context, since the Ghidra PR the semantics of the [blk]
operation in Core Theory was refined and corresponding theories were
updated. Before #1326 all theories were just ignoring the labels
passed to the [blk] operator. The reason for that was that the [blk]
operation serves two purposes. It enables concatenation of data and
control flow effects. And it attaches a label to that sequence so that
it could be referenced elsewhere. Before #1326 it was only used for
the first purpose, i.e., to merge data and control effects into the
single effect. But for Ghidra we needed an ability to create
labels (as ghidra is relying on Branch/Cbranch) instructions
everywhere, even to express the intra-instruction logic, not real
control flow.

Now, the theories have to take into account the label passed to the
blk operation, when they produce their denotations, unless the label is
Label.null. If the label is Label.null then the operation is
denotes just a sequence of data and control flow effects. Moreover,
denotations are allowed to coalesce several blocks together. But if
the label is non-null then the denotation has to preserve it.

Before this PR the BIR theory wasn't fully respecting the passed
labels and was sometimes optimizing them away, for example, when the
label was attached to an empty denotation. This PR takes care of
keeping the passed labels and at the same time preserving the minimal
form of the generated IR. Of course assuming that lifters are using
the blk operation correctly, i.e., that they are not passing
non-null labels to blocks that they do not plan to invoke later.

In other words, if you have a lifter that uses the blk operation you
need to update it and pass Theory.Label.null there instead of the
fresh label. This will have the same semantics as it had
before. Passing non-null label now has a different semantics.