Fix encoding and add parsing support for the arm/thumb CPS instruction:

Fix encoding and add parsing support for the arm/thumb CPS instruction:
- Add custom operand matching for imod and iflags.
- Rename SplitMnemonicAndCC to SplitMnemonic since it splits more than CC
  from mnemonic.
- While adding ".w" as an operand, don't change "Head" to avoid passing the
  wrong mnemonic to ParseOperand.
- Add asm parser tests.
- Add disassembler tests just to make sure it can catch all cps versions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125489 91177308-0d34-0410-b5e6-96231b3b80d8