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.

llvm-svn: 176955