[mips][sched] Split IIArith in preparation for the first scheduler targeting a specific MIPS CPU.

IIArith -> II_ADD, II_ADDU, II_AND, II_CL[ZO], II_DADDIU, II_DADDU,
  II_DROTR, II_DROTR32, II_DROTRV, II_DSLL, II_DSLL32, II_DSLLV,
  II_DSR[AL], II_DSR[AL]32, II_DSR[AL]V, II_DSUBU, II_LUI, II_MOV[ZFNT],
  II_NOR, II_OR, II_RDHWR, II_ROTR, II_ROTRV, II_SLL, II_SLLV, II_SR[AL],
  II_SR[AL]V, II_SUBU, II_XOR

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

This is necessary because the classes are shared between all schedulers.

Once this patch series is committed there will be an InstrItinClass for
each mnemonic with minimal grouping. This does increase the size of the
itinerary tables for each MIPS scheduler but we have a few options for dealing
with that later. These options include reducing the number of classes once
we see the best way to simplify them, or by extending tablegen to be able
to compress the table by eliminating duplicates entries, etc.