[Arm] Add assert in T2 Imm7s code emitter

[simonwallis2]

Summary

Add assert to provoke failure in object file output, not just in disassembly output.

[simonwallis2]

Ping?

[yroux]

LGTM Thanks.

llvm/lib/Target/ARM/MCTargetDesc/ARMMCCodeEmitter.cpp

if (!MO.isReg()) {
     isAdd = EncodeAddrModeOpValues(MI, OpIdx, Reg, Imm8, Fixups, STI);
 
   // FIXME: The immediate operand should have already been encoded like this
   // before ever getting here. The encoder method should just need to combine
   // the MI operands for the register and the offset into a single
   // representation for the complex operand in the .td file. This isn't just
   // style, unfortunately. As-is, we can't represent the distinct encoding
   // for #-0.
+  assert(((Imm8 & 0x3) == 0) && "Not a valid immediate!");
   uint32_t Binary = (Imm8 >> 2) & 0xff;
   // Immediate is always encoded as positive. The 'U' bit controls add vs sub.
   if (isAdd)
     Binary |= (1 << 8);
   Binary |= (Reg << 9);
   return Binary;
 }