Exercise art::arm::Thumb2Assembler::StoreToOffset for word pairs.

- Augment compiler/utils/arm/assembler_thumb2_test.cc.
- Ensure art::arm::Thumb2Assembler::StoreToOffset properly
  handles IP as (implicit) second source register.
- Remove the parity constraint on the first source register
  for art::arm::Thumb2Assembler::ldrd and
  art::arm::Thumb2Assembler::strd (as they are not required
  by the Thumb-2 encoding).
- Introduce additional versions of
  art::arm::Thumb2Assembler::ldrd and
  art::arm::Thumb2Assembler::strd accepting a second source
  register, which is not necessarily the one following the
  first source register, as it is allowed by the Thumb-2
  encoding.

Change-Id: I7dba168437a96a5cbb117058e9c547fb1ff5c295