[DAGCombiner] Fix fold (or (shuf A, V_0, MA), (shuf B, V_0, MB)) -> (shuf A,...

[DAGCombiner] Fix fold (or (shuf A, V_0, MA), (shuf B, V_0, MB)) -> (shuf A, B, Mask) to explicitly ensure that only one of the inputs of each shuffle is a zero vector.

This can only happen when we have a mix of zero and undef elements and the two vectors have a different arrangement of zeros/undefs. The shuffle should eventually be constant folded to all zeros.

Fixes PR32484.

llvm-svn: 299291