[ARM] Handle t2ADDri in ARMAsmPrinter::EmitUnwindingInstruction.

This fixes a bug in ARMAsmPrinter::EmitUnwindingInstruction where
llvm_unreachable was reached because t2ADDri wasn't handled.

Test case provided by Tim Northover.

rdar://problem/23270609

http://reviews.llvm.org/D14518

llvm-svn: 252557