ARM: correct an off-by-one in an assert

The assert was off-by-one, resulting in failures for valid input.

Thanks to Asiri Rathnayake for pointing out the failure!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224432 91177308-0d34-0410-b5e6-96231b3b80d8