[PATCH] [ARM] Add Thumb-2 code size optimization (wide CMP to narrow ADD)m

[Tim Northover]

> Yep, when I last thought about this I came to the conclusion
> (AArch64ISelLowering.cpp:1126) that it was valid provided imm != 0.

I now think INT_MIN/INT_MAX might be problematic too, but they're not
present here either.

Tim.