MC: address some comments in deprecation checks

Bob Wilson pointed out the unnecessary checks that had been committed to the
instruction check predicates.  The check was meant to ensure that the check was
not accidentally applied to non-ARM instructions.  This is better served as an
assertion rather than a condition check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224825 91177308-0d34-0410-b5e6-96231b3b80d8