[IROutliner] Adding instruction strings to IRSimilarityPrinting diagnostics.

When doing some recent debugging of the IROutliner, and using the similarity pass for debugging, just having the basic block and function isn't really enough to get all the information. This adds the first and last instruction to the output of the IRSimilarityPrinting pass to give better information to a user.

Reviewer: paquette

Differential Revision: https://reviews.llvm.org/D94304