cmd/compile: pick position of implicit break statements more carefully
The previous version used the position of the switch statement,
which makes for potentially jumpy stepping and introduces a large
number of statements repeating the line (tricky for inserting
breaks). It also shared a single OBREAK node and this was not
really a syntax "tree".
This improves both the nostmt test (by 6 lines) and
reduces the total badness score from dwarf-goodness (by about 200).
Change-Id: I1f71b231a26f152bdb6ce9bc8f95828bb222f665
Reviewed-on: https://go-review.googlesource.com/c/go/+/188218
Run-TryBot: David Chase <drchase@google.com> Reviewed-by: Jeremy Faller <jeremy@golang.org>