Avoid false unreachable and redundant-expr warnings in loops more robustly and efficiently, and avoid multiple revealed type notes for the same line.

[tyralla]

Fixes #18606
Closes #18511
Improves #18991

This change is an improvement over 9685171. Besides fixing the regression reported in #18606 and removing the duplicates reported in #18511, it should significantly reduce the performance regression reported in #18991. At least running Measure-command {python runtests.py self} on my computer (with removed cache) is 10 % faster.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[tyralla]

@JukkaL: I thought that later responses to reveal_type are always better than earlier ones. However, this is not the case for testNewRedefineTryStatement test case. Maybe the most general approach would be to collect all the union items of the individual responses and to combine them in the final note. I could try this, if you think it's a good way to go.

Alternatively, I could just remove the reveal_type part of this pull request, as it seems less important than the other two addressed issues.