[mips] Clean up MipsISelDAGToDAG.cpp and MipsISelLowering.cpp.

- Rename function.
- Pass iterator by value.
- Remove header include.

No functionality changes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179312 91177308-0d34-0410-b5e6-96231b3b80d8