ARM: remove invalid invalid tests

The tests were checking for barriers which the ARM ARM says they must execute
as a full system DMB/DSB, rather than that they're UNDEFINED and LLVM does in
fact represent them.

The tests happened to be passing because they were using a non-versioned ARM
triple which didn't have *any* DMB/DSB instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186662 91177308-0d34-0410-b5e6-96231b3b80d8