[DAGCombine] Fix a bug in a BUILD_VECTOR combine

When trying to convert a BUILD_VECTOR into a shuffle, we try to split a single source vector that is twice as wide as the destination vector. 
We can not do this when we also need the zero vector to create a blend.
This fixes PR22774.

Differential Revision: http://reviews.llvm.org/D8040

llvm-svn: 231219