ARM: move llvm_unreachable use
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@207609 91177308-0d34-0410-b5e6-96231b3b80d8

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