[Bug middle-end/4210] should not warning with dead code

[eggert at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=4210

--- Comment #25 from Paul Eggert <eggert at gnu dot org> ---
I'd like this bug to be changed from SUSPENDED to CONFIRMED, given that it's
continuing to be a problem (e.g., bug#79479).

Also, I'd like to suggest what I hope is a simple fix. In 2006 Joseph wrote
"skip_evaluation can't be set for if (0) because you can jump into if (0),
whereas jumps into statement expressions are not permitted". So, how about if
we merely set skip_evaluation for "if (0)" when the then-part lacks labels?
This should be an easy test, as it shouldn't require parsing the whole function
body. The test might still generate false alarms for code containing gotos, but
in practice such gotos are rare, so the proposed change should be a significant
improvement even if it's not perfect.