[ARM] Do not generate invalid encoding for stack adjust, even if this is just

temporary.

Because of that:
1. The machine verifier was complaining on such code.
2. The generate code worked just because the thumb reduction size pass fixed the
opcode.

rdar://problem/20749824


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236247 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/Thumb2InstrInfo.cpp

@@ -256,14 +256,19 @@ void llvm::emitT2RegPlusImmediate(MachineBasicBlock &MBB,
     if (Fits) {
       if (isSub) {
         BuildMI(MBB, MBBI, dl, TII.get(ARM::t2SUBrr), DestReg)
-          .addReg(BaseReg, RegState::Kill)
+          .addReg(BaseReg)
           .addReg(DestReg, RegState::Kill)
           .addImm((unsigned)Pred).addReg(PredReg).addReg(0)
           .setMIFlags(MIFlags);
       } else {
+        // Here we know that DestReg is not SP but we do not
+        // know anything about BaseReg. t2ADDrr is an invalid
+        // instruction is SP is used as the second argument, but
+        // is fine if SP is the first argument. To be sure we
+        // do not generate invalid encoding, put BaseReg first.
         BuildMI(MBB, MBBI, dl, TII.get(ARM::t2ADDrr), DestReg)
+          .addReg(BaseReg)
           .addReg(DestReg, RegState::Kill)
-          .addReg(BaseReg, RegState::Kill)
           .addImm((unsigned)Pred).addReg(PredReg).addReg(0)
           .setMIFlags(MIFlags);
       }