cmd/internal/gc: clean up switch code

This CL makes the switch walking and typechecking code
more idiomatic and adds documentation.
It also removes all but one global variable.

No functional changes. Confirmed with toolstash -cmp on the stdlib.

Change-Id: Ic3f38acc66e906edd722498839aeb557863639cf
Reviewed-on: https://go-review.googlesource.com/6268Reviewed-by: Russ Cox <rsc@golang.org>