Remove non-existent ARM insn kThumb2SubsRRI12.

For kOpSub/kOpAdd, prefer modified immediate encodings
because they set flags.

Change-Id: I41dcd2d43ba1e62120c99eaf9106edc61c41e157
diff --git a/compiler/dex/quick/arm/int_arm.cc b/compiler/dex/quick/arm/int_arm.cc
index 43928fc..1e290ad 100644
--- a/compiler/dex/quick/arm/int_arm.cc
+++ b/compiler/dex/quick/arm/int_arm.cc
@@ -753,7 +753,8 @@
 // Decrement register and branch on condition
 LIR* ArmMir2Lir::OpDecAndBranch(ConditionCode c_code, int reg, LIR* target) {
   // Combine sub & test using sub setflags encoding here
-  NewLIR3(kThumb2SubsRRI12, reg, reg, 1);
+  OpRegRegImm(kOpSub, reg, reg, 1);  // For value == 1, this should set flags.
+  DCHECK(last_lir_insn_->u.m.def_mask & ENCODE_CCODE);
   return OpCondBranch(c_code, target);
 }