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>
3 files changed
tree: 45ac588f3e15ab08ed9efeac1f6e4e3d540ba769
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
  18. SECURITY.md
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