Generate simple scalar expressions unconditionally

In many cases it would be simplest and most efficient to generate a series of conditional attributes as if they were procedural code:

x1 := IF(c1, b, c);
x2 := IF(c2, x1, ...);
x3 := IF(c3, x1, x2);

Which would avoid some of the combinatorial explosion in code size.

As an initial version assume all external functions and dataset operations are costly.