R600/SI: Fix insertelement with dynamic indices.

This didn't work for any integer vectors, and didn't
work with some sizes of float vectors. This should now
work with all sizes of float and i32 vectors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200619 91177308-0d34-0410-b5e6-96231b3b80d8