Continues outside of a loop are not allowed. Most of these possibilities
were tested in label1.go, but one was missing - a plain continue in a
switch/select but no enclosing loop.

This used to error with a "continue not in loop" in 1.8, but recently
was broken by c03e75e5. In particular, innerloop does not only account
for loops, but also for switches and selects. Swap it by bools that
track whether breaks and continues should be allowed.

While at it, improve the wording of errors for breaks that are not where
they should be. Change "loop" by "loop, switch, or select" since they
can be used in any of those.

And add tests to make sure this isn't broken again. Use a separate func
since I couldn't get the compiler to crash on f() itself, possibly due
to the recursive call on itself.

Fixes #19934.

Change-Id: I8f09c6c2107fd95cac50efc2a8cb03cbc128c35e
Reviewed-on: https://go-review.googlesource.com/40357
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>