Special case aliases in GlobalValue::getAlignment.

An alias has the address of what it points to, so it also has the same
alignment.

This allows a few optimizations to see past aliases for free.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208103 91177308-0d34-0410-b5e6-96231b3b80d8