Add a getAlignOf helper for getting the ABI alignment of a

Add a getAlignOf helper for getting the ABI alignment of a
type as a target independent constant expression.  I confess
that I didn't check that this method works as intended (though
I did test the equivalent hand-written IR a little).  But what
could possibly go wrong!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72213 91177308-0d34-0410-b5e6-96231b3b80d8