[llvm-reduce] Fix di-metadata pass test failures

We're seeing intermittent failures in upstream bots. See:

https://lab.llvm.org/buildbot/#/builders/139/builds/29185
https://lab.llvm.org/buildbot/#/builders/238/builds/295

This appears to be due to the unstable iteration order of DenseSet.
Since we're trying to reduce a tree, it makes sense to attempt
reductions from the top down.

This also addresses post-review comments from @MatzeB.

Differential Revision: https://reviews.llvm.org/D135473