AMDGPU: Report extractelement as free in cost model

AMDGPU: Report extractelement as free in cost model

The cost for scalarized operations is computed as N * (scalar operation
cost + 1 extractelement + 1 insertelement). This partially fixes
inflating the cost of scalarized operations since every operation is
scalarized and free. I don't think we want any cost asociated with
scalarization, but for now insertelement is still counted. I'm not sure
if we should pretend that insertelement is also free, or add a way
to compute a custom scalarization cost.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254438 91177308-0d34-0410-b5e6-96231b3b80d8