R600/SI: Add failing test for 3 x i64 vectors.

Stores of <4 x i64> do work (although they do expand to 4 stores
instead of 2), but 3 x i64 vectors fail to select.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200989 91177308-0d34-0410-b5e6-96231b3b80d8