[mips][sched] Split IIHiLo into II_MFHI_MFLO and II_MTHI_MTLO

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



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199493 91177308-0d34-0410-b5e6-96231b3b80d8