`DT` is always the analysis for the to-be-optimized function while
`GenDT` is the analysis of the function that we currently generate code
for, which can also be an outlined function. Here, we want to check
dominance in the generated code, hence we must use `GenDT`.
#179433 already fixed the same issue for `BlockGenerator`. The same
pattern is used in `RegionGenerator` which is fixed here. A good
argument to avoid code duplication.
Fixes: #185313
Thanks to @jaschiu for the bug report and reproducer
42b4dce07c