MMX is obsolete.

SSE/AVX has its own set of registers. It can do floating point and integer operations. It would not be correct to refer to it as a floating point unit.

GPUs are good at mining because they can do so many calculations in parallel. They're just as capable of integer operations as the SIMD units on a CPU.

MMX was integer SIMD that re-used the FPU register file, but was not doing floating-point operations.