cmd/compile: refine statement marking in numberlines

1) An empty block is treated as not-a-statement unless its line differs
from at least one of its predecessors (it might make sense to
rearrange branches in predecessors, but that is a different issue).

2) When iterating forward to choose a "good" place for a statement,
actually check that the chosen place is in fact good.

3) Refactor same line and same file into methods on XPos and Pos.

This reduces the failure rate of ssa/stmtlines_test by 7-ish lines.
(And interacts favorably with later debugging CLs.)

Change-Id: Idb7cca7068f6fc9fbfdbe25bc0da15bcfc7b9d4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/188217


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