1. A data processing apparatus comprising: processing circuitry configured to perform a data processing operation in response to a micro-operation, the processing circuitry having an output register configured to store a result of the data processing operation; control circuitry configured to detect whether a current micro-operation to be processed by the processing circuitry would give the same result as the last valid micro-operation to be processed by the processing circuitry before said current micro-operation; wherein if the control circuitry detects that said current micro-operation would give the same result as said last valid micro-operation, then the control circuitry is configured to prevent the processing circuitry from processing said current micro-operation so that the output register is not updated in response to said current micro-operation, and to control the processing circuitry to output the result stored in the output register as the result of said current micro-operation; and a table comprising at least one table entry, each table entry configured to identify an earlier micro-operation and the result of the earlier micro-operation; wherein the control circuitry is configured to perform a table lookup to detect whether the table comprises a corresponding table entry identifying an earlier micro-operation which would give the same result as said current micro-operation; and wherein if the control circuitry detects that said current micro-operation would give the same result as said last valid micro-operation, then the control circuitry is configured to omit the table lookup.