[DAGCombine] combineShuffleOfScalars - handle non-zero SCALAR_TO_VECTOR indices (PR41097)

rL356292 reduces the size of scalar_to_vector if we know the upper bits are undef - which means that shuffles may find they are suddenly referencing scalar_to_vector elements other than zero - so make sure we handle this as undef.

llvm-svn: 356327