[IRSim] Check largest sections first when analyzing similarity

When we check for similarity, right now there is no order to how it is checked, except for via the suffix tree ordering.

We can reduce how much structural analysis we perform by checking the the regions in decreasing size. In doing so, we know that if two large sections match, each of their contained regions also match. This allows us to skip the structural checking for each smaller section. IT does require that we use the large regions as a "bridge" to create the canonical mapping between the two regions.

This reduces compile time significantly for some benchmarks. It will not perform as well for programs with many small items.

Recommit fixes the IRSimilarity tests.

Recommit of: 805ec19d

Reviewer: paquette
Differential Revision: https://reviews.llvm.org/D139338