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
diff --git a/compiler/utils/arm/assembler_thumb2.h b/compiler/utils/arm/assembler_thumb2.h
index 81dd138..e33c240 100644
--- a/compiler/utils/arm/assembler_thumb2.h
+++ b/compiler/utils/arm/assembler_thumb2.h
@@ -135,9 +135,17 @@
   void ldrsb(Register rd, const Address& ad, Condition cond = AL) OVERRIDE;
   void ldrsh(Register rd, const Address& ad, Condition cond = AL) OVERRIDE;
 
+  // Load/store register dual instructions using registers `rd` and `rd` + 1.
   void ldrd(Register rd, const Address& ad, Condition cond = AL) OVERRIDE;
   void strd(Register rd, const Address& ad, Condition cond = AL) OVERRIDE;
 
+  // Load/store register dual instructions using registers `rd` and `rd2`.
+  // Note that contrary to the ARM A1 encoding, the Thumb-2 T1 encoding
+  // does not require `rd` to be even, nor `rd2' to be equal to `rd` + 1.
+  void ldrd(Register rd, Register rd2, const Address& ad, Condition cond);
+  void strd(Register rd, Register rd2, const Address& ad, Condition cond);
+
+
   void ldm(BlockAddressMode am, Register base,
            RegList regs, Condition cond = AL) OVERRIDE;
   void stm(BlockAddressMode am, Register base,