Clean up the IN operator code generation logic to make it easier to reason
about.  In the process, improve code generation to omit some unused OP_Null
operations.
Remove an unnecessary OP_Null in the IN-operator logic.  Attempt to clarify
comments explaining the IN-operator code, though it is not clear that the
comments are correct even yet - more work to be done.
Improved detection and handling of NULL values on the RHS of a IN operator.
A better comment on the generated code for the NULL-in-RHS-of-IN detection
logic.
Remove (newly) incorrect preprocessor check to fix build on WinRT.
Enhancements to the code generator for the IN operator that result in much
faster queries in some cases, for example when the RHS of the IN operator
changes for each row of a large table scan.