Cranelift: build_value_labels_ranges makes several incorrect assumptions

* It assumes only one new `ValueLabel` introduced at each `SourceLoc`. (`build_value_labels_index`)
* It assumes `SourceLoc` is strictly ascending. (Checking for `last_srcloc.unwrap() > srcloc` instead of `last_srcloc.unwrap() != srcloc`)
* It assumes that a `ValueLabel` is only alive for a continuous range of `SourceLoc`. (`values_labels.range(range)`)

All of those assumptions are broken by cg_clif, meaning that `build_value_labels_ranges` often misses values in the output.