`BoundedValue`: support for multiple ranges

This commit introduces radically changes the implementation of
`BoundedValue`: it no longer represents a single, contiguous range, but
an arbitrary number of ranges.

The bounds are now represented through a
`llvm::SmallVector<std::pair<uint64_t, uint64_t>, 3>`.

* Introduce the `BoundedValue::bounds()` method, which allows to iterate
  over all the ranges that a `BoundedValue` represents. The `bounds`
  method returns a `Bounds` object, which can be used as a range
  composed by `BoundsIterator`.
* All the methods dealing with the `BoundedValue`'s bounds have been
  rewritten.
* New debugging information: "bv-merge". Print all the computations
  performed by `BoundedValue::mergeImpl`.
* Drop dead code: `BoundedValue::setBound` and `isPositive`
* Introduce `BoundedValue::isRightOpen` and drop
  `BoundedValue::isSingleRange`