[TTI] Start process of merging OperandValueKind and OperandValueProperties [nfc]

OperandValueKind and OperandValueProperties both provide facts about the operands of an instruction for purposes of cost modeling.  We've discussed merging them several times; before I plumb through more flags, let's go ahead and do so.

This change only adds the client side interface for getArithmeticInstrCost and makes a couple of minor changes in client code to prove that it works.  Target TTI implementations still use the split flags.  I'm deliberately splitting what could be one big change into a series of smaller ones so that I can lean on the compiler to catch errors along the way.