[Bug tree-optimization/78895] [6 Regression] wrong code with -O1 when setting union twice since 6.3

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78895

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Duplicate of PR71055 (missed backport of a latent bug).