PCie and CPU connectors both use 12V and ground. CPU is 4x 12V and PCIe is 3x 12V (the +2 on PCIe are just sense wires). You cannot plug a 4+4 CPU connector directly in to a graphics card, or a PCIe connector in to a motherboard. CPU has the 12V across the top row and PCIe is across the bottom row.
Because it's just 12V and ground wires it's possible to use the same connector on the power supply for both CPU and PCIe - Though not all power supplies do this.
It's possible to use an adapter a 4+4pin CPU connector to a PCIe connector, though I normally wouldn't recommend adding additional adapters as it just adds additional failure points and increases resistance across the cable. Typically speaking power supplies will have more PCIe cables than CPU power cables anyway. Most power supplies only have a single 4+4 connector, though some higher wattage models will come with two. Typically though PSUs with 2 CPU cables will have at least 2 PCIe cables as well so unless you're powering multiple GPUs you shouldn't really have any need to adapt a CPU connector to PCIe.
With modular power supplies you could always just get another modular cable compatible with your model of power supply. The CS750M that OP has uses Corsair's Type 3 cables which are also compatible with Corsair's Type 4 PCIe and CPU power cables.
