Linear types: remember usage environment of a case
Every Core expression has a well-defined type (exprType). With linear types, it should likewise have a well-defined usage environment.
For case expressions, the usage environment is computed by checking all branches and taking sup. However, this trick doesn't work when there are no branches.
Current workaround: empty case has a special Bottom usage, which denotes an arbitrary usage (0, 1, Many).
Wart: this means that we have to overapproximate, e.g. in scaleUsage x Bottom we have to guess whether to return x or Zero and we cannot take infimum.
Desired solution:
(1) Just like a case expression remembers its type (Note [Why does Case have a 'Type' field?] in Core.hs), it should remember its usage environment. This data should be verified by Lint.
(2) Once this is done, we can remove the Bottom usage and the second field of UsageEnv. In this step, we have to infer the correct usage environment for empty case in the typechecker.