R600: Use EXPORT_RAT_INST_STORE_DWORD for stores on Cayman

R600: Use EXPORT_RAT_INST_STORE_DWORD for stores on Cayman

We were using RAT_INST_STORE_RAW, which seemed to work, but the docs
say this instruction doesn't exist for Cayman, so it's probably safer
to use a documented instruction instead.

Reviewed-by: Vincent Lejeune<vljn at ovi.com>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184015 91177308-0d34-0410-b5e6-96231b3b80d8