Current definition of subtract with immediate instruction aliases uses CodeGenOnly...

Current definition of subtract with immediate instruction aliases uses CodeGenOnly defined instructions and post matcher expansion methods to emit real instructions add with immediate. However, they can directly alias add with immediate instruction and remove unnecessary definitions and code in MipsAsmParser.cpp. This patch makes no change in functionality, just removes unnecessary definitions and code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206272 91177308-0d34-0410-b5e6-96231b3b80d8