1. Apparatus for processing data comprising: data processing circuitry configured to execute data processing operations specified by program instructions; and instruction decoder circuitry configured to decode program instructions to generate control signals for controlling said data processing circuitry to execute said data processing operations specified by said program instructions; wherein said instruction decoder circuitry and said data processing circuitry are configured to decode and to execute a sequence of program instructions to evaluate a data processing function; said sequence of program instructions includes an intermediate value generating instruction for controlling generation of an intermediate value including an intermediate data value and an embedded opcode value; said sequence of program instructions includes, after said intermediate value generating instruction, an intermediate value consuming instruction for consuming said intermediate value; said instruction decoder circuitry and said data processing circuitry are configured to operate in dependence upon said embedded opcode value to follow one of a first execution path and a second execution path: (i) said first execution path comprising completing evaluation of said data processing function using said intermediate value consuming instruction; and (ii) said second execution path comprising substituting a substitute instruction in place of said intermediate value consuming instruction and executing said substitute instruction to evaluate said data processing function instead of said sequence of program instructions.