[mips] Delete unnecessary code.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192660 91177308-0d34-0410-b5e6-96231b3b80d8
[mips] Set HI/LO registers' HWEncoding field.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192661 91177308-0d34-0410-b5e6-96231b3b80d8