[MachineOutliner] Efficient Implementation of MachineOutliner::findCandidates() (#90260)

This reduce the time complexity of the main loop of `findCandidates()`
method from $O(n^2)$ to $O(n \log n)$.

For small $n$, the modification does not regress the build time, but it
helps significantly when $n$ is large.

For one application, this reduces the runtime of the main loop from 120
seconds to 28 seconds.

This is the first commit for an enhanced version of machine outliner --
see
[RFC](https://discourse.llvm.org/t/rfc-enhanced-machine-outliner-part-1-fulllto-part-2-thinlto-nolto-to-come/78732).