ARM cost model: Penalize insertelement into D subregisters

Swift has a renaming dependency if we load into D subregisters. We don't have a
way of distinguishing between insertelement operations of values from loads and
other values. Therefore, we are pessimistic for now (The performance problem
showed up in example 14 of gcc-loops).

radar://13096933

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174300 91177308-0d34-0410-b5e6-96231b3b80d8