Make custom lowering of ADD work correctly. This

fixes PR2476; patch by Richard Osborne.  The same
problem exists for a bunch of other operators, but
I'm ignoring this because they will be automagically
fixed when the new LegalizeTypes infrastructure lands,
since it already solves this problem centrally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52610 91177308-0d34-0410-b5e6-96231b3b80d8