[DAGCombiner] visitSIGN_EXTEND_INREG should fold sext_vector_inreg(undef) to 0 not undef.

We need to ensure that the sign bits of the result all match
so we can't fold to undef.

Similar to PR46585.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D83163