Similar to bitcast, treat addrspacecast as a foldable operand.

Added a test sink-addrspacecast.ll to verify this change.

Patch by Jingyue Wu.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209343 91177308-0d34-0410-b5e6-96231b3b80d8