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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299291 91177308-0d34-0410-b5e6-96231b3b80d8