Reapply r128585 without generating a lib depedency cycle. An updated log:

- Implement asm parsing support for LDRT, LDRBT, STRT, STRBT and
  {STR,LDC}{2}_{PRE,POST} fixing the encoding wherever is possible.
- Move all instructions which use am2offset without a pattern to use
  addrmode2.
- Add a new encoding bit to describe the index mode used and teach
  printAddrMode2Operand to check by the addressing mode which index
  mode to print.
- Testcases

llvm-svn: 128632