[mips] Define two subclasses of MipsDAGToDAGISel. Mips16DAGToDAGISel is for
mips16 and MipsSEDAGToDAGISel is for mips32/64. 

No functionality changes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177089 91177308-0d34-0410-b5e6-96231b3b80d8
Add header file MipsISelDAGToDAG.h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177090 91177308-0d34-0410-b5e6-96231b3b80d8