The range analysis wasn't producing useful bounds for `AssignOperation`s
(`+=`, `-=`) unless their RHS involved a variable. This is because a
shortcut was made in the `analyzableDef` predicate, which used to
specify that an analyzable definition was one for which we'd specified
the dependencies. But we can't distinguish between having _no
dependencies_ and having _no specification of the dependencies_.
The fix is to be more explicit about which definitions are analyzable.
To avoid too much repetition I'm still calling out to `analyzableExpr`
in the new code.
The pretty-printing of a QL `float` didn't include enough digits to tell
whether a large number had accurate bounds. The `toString` value of a
float appears to be more precise.
