[IRSim] Adding IR Instruction Mapper

This introduces the IRInstructionMapper, and the associated wrapper for
instructions, IRInstructionData, that maps IR level Instructions to
unsigned integers.

Mapping is done mainly by using the "isSameOperationAs" comparison
between two instructions.  If they return true, the opcode, result type,
and operand types of the instruction are used to hash the instruction
with an unsigned integer.  The mapper accepts instruction ranges, and
adds each resulting integer to a list, and each wrapped instruction to
a separate list.

At present, branches, phi nodes are not mapping and exception handling
is illegal.  Debug instructions are not considered.

The different mapping schemes are tested in
unittests/Analysis/IRSimilarityIdentifierTest.cpp

Recommit of: b04c1a9d

Differential Revision: https://reviews.llvm.org/D86968