mir: Reinstate while loop in deaggregator pass

mir: Reinstate while loop in deaggregator pass

A previous commit must have removed the `while let` loop here by
mistake; for each basic block, it should find and deaggregate multiple
statements in their index order, and the `curr` index tracks the
progress through the block.

This fixes both the case of deaggregating statements in separate
basic blocks (preserving `curr` could prevent that) as well
as multiple times in the same block (missing loop prevented that).