ARM: improve instruction validation for thumb mode

The ARM Architecture Reference Manual states the following:
  LDM{,IA,DB}:
    The SP cannot be in the list.
    The PC can be in the list.
    If the PC is in the list:
      • the LR must not be in the list
      • the instruction must be either outside any IT block, or the last
        instruction in an IT block.
  POP:
    The PC can be in the list.
    If the PC is in the list:
      • the LR must not be in the list
      • the instruction must be either outside any IT block, or the last
        instruction in an IT block.
  PUSH:
    The SP and PC can be in the list in ARM instructions, but not in Thumb
    instructions.
  STM:{,IA,DB}:
    The SP and PC can be in the list in ARM instructions, but not in Thumb
    instructions.

llvm-svn: 224502