[llvm] [LowerSwitch] Don't let pass manager handle the dependency (PR #68662)

[Jay Foad via llvm-commits]

jayfoad wrote:

> But the legacy PM treats dependent analyses and passes the same way, so you'd have to rewrite the legacy PM

That's obviously not gonna happen. But just because the legacy PM _supports_ using a pass like an analysis, that does not mean we have _use_ it like that in this case.

> The change here will help remove one extra run of this pass for AMDGPU

That is a clear win. The question is: do we want to do it like in this patch? Or by declaring that more passes preserve LowerSwitch as in https://github.com/jayfoad/llvm-project/commit/c03cbd4d2f584d24e2100fee529f2ba1d18c345d.

https://github.com/llvm/llvm-project/pull/68662