The MachineFunction should have been created with the correct subtarget. As
long as there is no way to change it, MipsTargetMachine can just capture it
directly from the MachineFunction without calling getSubtargetImpl again.
While there, const correct the Subtarget pointer to avoid a const_cast.
I believe the Mips16Subtarget and NoMips16Subtarget members are never used, but
I'll leave there removal for a separate patch.