[IRSim] Strip out the findSimilarity call from the constructor

Both doInitialize and runOnModule were running the entire analysis
due to the actual work being done in the constructor. Strip it out here
and only get the similarity during runOnModule.

Author: lanza
Reviewers: AndrewLitteken, paquette, plofti

Differential Revision: https://reviews.llvm.org/D92524