ARM: move llvm_unreachable use

When building with -Werror=covered-switch-default (as on the buildbots), the
build would fail since all cases are covered by the switch.  Move the
llvm_unreachable to the end of the function as an annotation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207609 91177308-0d34-0410-b5e6-96231b3b80d8