[SDAG switch lowering] Fix switch case -> or merging for 0 and INT_MIN

The big/small ordering here is based on signed values so SmallValue will
be INT_MIN and BigValue 0. This shouldn't be a problem but the code
assumed that BigValue always had more bits set than SmallValue.

We used to just miss the transformation, but a recent refactoring of
mine turned this into an assertion failure.

llvm-svn: 239105