Make bitcast, extractelement, and insertelement considered cheap for speculation.

This helps more branches into selects. On R600,
vectors are cheap and anything that helps
remove branches is very good.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209914 91177308-0d34-0410-b5e6-96231b3b80d8