[mips] Rename functions. No functionality changes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181713 91177308-0d34-0410-b5e6-96231b3b80d8
[mips] Define a helper function which creates an instruction with the same
operands as the prototype instruction but with a different opcode.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181714 91177308-0d34-0410-b5e6-96231b3b80d8