[IRSim] Adding IRSimilarityCandidate that contains a region of IRInstructionData.

The IRSimilarityCandidate is a container to hold a region of
IRInstructions and offer interfaces for the starting instruction, ending
instruction, parent function, length.  It also assigns a global value
number for each unique instance of a value in the region.

It also contains an interface to compare two IRSimilarity as to whether
they have the same sequence of similar instructions.

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

Recommit of: 4944bb19

Differential Revision: https://reviews.llvm.org/D86970