[IRSim] Adding structural comparison to IRSimilarityCandidate.

Just because sequences of instructions are similar to one another,
doesn't mean they are doing the same thing.

This introduces a structural check for the IRSimilarityCandidate that
compares two IRSimilarityCandidates against one another, and in each
instruction creates a mapping between the operands and results, or
checks that the existing mapping is valid.  If this check passes, it
means we have structurally similar IRSimilarityCandidates.

Tests for whether the candidates are found in
unittests/Analysis/IRSimilarityIdentifierTest.cpp.

Recommit of: b27db2bb for Differential
URL.

Differential Revision: https://reviews.llvm.org/D86971