#468 More precise notUsed() for register swap

[Philipp Klaus Krause]

notUsed() currently considers all register swap instructions, such as ex de, hl on Z80 to read both registers. For a more exact notUsed(), we could try to track the registers more exactly (i.e. when looking for reads of l, we would look for reads of e from the ex on). This probably requires to replace the 1-bit bit-field visited in lineNode to be replaced by a mask with as many bits as we have registers.

Philipp