Register Allocator: refactoring and add comments.

No functionality change. Thanks Andy for reviewing.

rdar://16162005


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204962 91177308-0d34-0410-b5e6-96231b3b80d8
[PowerPC] Expand v2i64 shifts

These operations need to be expanded during legalization so that isel does not
crash. In theory, we might be able to custom lower some of these. That,
however, would need to be follow-up work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204963 91177308-0d34-0410-b5e6-96231b3b80d8