ARM cost model: Add test case to make sure we would notice a change in CodeGen

In r176898 I updated the cost model to reflect the fact that sext/zext/cast on
v8i32 <-> v8i8 and v16i32 <-> v16i8 are expensive.

This test case is so that we make sure to update the cost model once we fix
CodeGen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176955 91177308-0d34-0410-b5e6-96231b3b80d8