[mips] Define two subclasses of MipsTargetLowering. Mips16TargetLowering is for
mips16 and MipsSETargetLowering is for mips32/64. 

No functionality changes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176917 91177308-0d34-0410-b5e6-96231b3b80d8