cmd/compile: modify line number printing for nodes

add preceding "+" for a line that is certainly a statement,
and "_" for a line this is certainly not a statement.

Change-Id: I831435dbc2302f25db1320b99d3513fe61fe1fa2
Reviewed-on: https://go-review.googlesource.com/c/go/+/198737


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>