Reduce cost of branching

[Schaeff]

Abstract

Reduce the cost of c ? l : r

Motivation

Constraint count for branching is not optimal

Specification

Change code generation for c ? l : r from

ll == c * l
rr == (1 - c) * r
res == ll + rr

to

# res := c ? l : r
res - r = c * (l - r)

Backwards Compatibility

This is backwards compatible, though we need to check that there are no regressions in any of the optimiser steps