[IRSim] Adding a basic similarity identifier.

This takes the mapped instructions from the IRInstructionMapper, and
passes it to the Suffix Tree to find the repeated substrings.  Within
each set of repeated substrings, the IRSimilarityCandidates are compared
against one another for structure, and ensuring that the operands in the
instructions are used in the same way.  Each of these structurally
similarity IRSimilarityCandidates are contained in a SimilarityGroup.

Tests checking for identifying identity of structure, different
isomorphic structure, and different
nonisomoprhic structure are found in
unittests/Analysis/IRSimilarityIdentifierTest.cpp.

Differential Revision: https://reviews.llvm.org/D86972