Lift non-immediate expressions out of case values for the Nockma backend

[lukaszcz]

Implements a transformation compute-case-anf which lifts out non-immediate values matched on in case expressions by introducing let-bindings for them. In essence, this is a partial ANF transformation for case expressions only.

For example, transforms

case f x of { c y := y + x; d y := y }

to

let z := f x in case z of { c y := y + x; d y := y }

This transformation is needed to avoid duplication of values matched on in case-expressions in the Nockma backend.