R600: Change how vector truncating stores are packed.

Don't introduce new operations on an illegal sub 32-bit type.
Do the operations on a 32-bit value, and then use a truncating store.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206864 91177308-0d34-0410-b5e6-96231b3b80d8