Correct ARM NOP encoding

The encoding of NOP in ARMAsmBackend.cpp is missing a trailing zero, which
causes the emission of a coprocessor instruction rather than "mov r0, r0"
as indicated in the comment.  The test also checks for the wrong encoding.

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20121203/157919.html

llvm-svn: 169420