also fix the encoding of the later.
- Add a new encoding bit to describe the index mode used in AM3.
- Teach printAddrMode3Operand to check by the addressing mode which
index mode to print.
- Testcases.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128832 91177308-0d34-0410-b5e6-96231b3b80d8
