cmd/compile: suppress statement marks on interior of switch tree

The lines on nodes within the IF-tree generated for switch
statements looks like control flow so the lines get marked
as statement boundaries.  Except for the first/root comparison,
explicitly disable the marks.

Change-Id: I64b966ed8e427cdc6b816ff6b6a2eb754346edc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/198738


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