Sink AddrMode back into TargetLowering, removing one of the most
peculiar headers under include/llvm.

This struct still doesn't make a lot of sense, but it makes more sense
down in TargetLowering than it did before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171739 91177308-0d34-0410-b5e6-96231b3b80d8