[mips][sched] Split IIImul and IIImult into subclasses.

IIImul -> II_MUL
IIImult -> II_MULT, II_MULTU, II_MADD, II_MADDU, II_MSUB, II_MSUBU, II_DMULT, II_DMULTU

No functional change since the InstrItinData's have been duplicated.