cmd/cover: fix sorting of profile segment boundaries, again

This is a refinement of CL 114855, which fixed the empty clause case,
but broke some other cases where segment boundaries can coincide
for other reasons.

Fixes #25767.

Change-Id: I2a387c83f9d651c8358f3e11b03f6167af0eb8bf
Reviewed-on: https://go-review.googlesource.com/116976


Run-TryBot: David Symonds <dsymonds@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>