Add a couple of cases to mk_switch to catch the case when we have a
tag range that has a lot of default cases at either end, and we're not
using a single switch.  In situations like this we want to eliminate
the default cases with an if-statement, before dealing with the rest
of the branches, which might then be suitable for a switch.

Also, ignore empty tag slots at either end of the range if there is no
default case.

This might work around a gcc 2.95 bug that we tickled with the
code being generated before.