match lowering: don't collect test alternatives ahead of time
I'm very happy with this one. Before this, when sorting candidates into the possible test branches, we manually computed `usize` indices to determine in which branch each candidate goes. To make this work we had a first pass that collected the possible alternatives we'd have to deal with, and a second pass that actually sorts the candidates.
In this PR, I replace `usize` indices with a dedicated enum. This makes `sort_candidates` easier to follow, and we don't need the first pass anymore.
r? ``@matthewjasper``
