Add a new Operator class, for handling Instructions and ConstantExprs

Add a new Operator class, for handling Instructions and ConstantExprs
in a convenient manner, factoring out some common code from
InstructionCombining and ValueTracking. Move the contents of
BinaryOperators.h into Operator.h and use Operator to generalize them
to support ConstantExprs as well as Instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@76232 91177308-0d34-0410-b5e6-96231b3b80d8