Handle unreachable code in different ways

Handle unreachable code in different ways

The idea is to remove unreachable code where possible. To change it to
asserts where it makes sense. To mark it with explicit assert(false)
where unreachable code can't be removed.

There are cases where unreachable code needs to exist to silence
compiler warnings. Eg. default cases for switch. Mark them with
assert(false) so it's immediately visible that the flow should never
reach them (e.g. because the check has been performed earlier and it's
internal function).

Change-Id: I7d53c9772fe54b5c4dfd0f7205eec633fe4b9c20