cmd/compile: use a map to track const switch cases
This is simpler than the sorting technique.
It also allows us to simplify or eliminate
some of the sorting decisions.
Most important, sorting will not work when case clauses
represent ranges of integers: There is no correct
sort order that allows overlap detection by comparing
neighbors. Using a map allows of a cheap, simple
approach to ranges, namely to insert every int
in the map. The equivalent approach for sorting
means juggling temporary Nodes for every int,
which is a lot more expensive.