[release_36] Cherry-pick r231219.

Original message:
[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_36@232807 91177308-0d34-0410-b5e6-96231b3b80d8