R600: Implement TargetLowering::getVectorIdxTy()

We use MVT::i32 for the vector index type, because we use 32-bit
operations to caculate offsets when dynamically indexing vectors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187749 91177308-0d34-0410-b5e6-96231b3b80d8