[DAG] Fold extract_vector_elt (scalar_to_vector), K to undef (K != 0)

This was unconditionally folding this to the source operand, even if the access was out of bounds. Use undef instead of the extract is not the first element.

This helps with some cases where 3-vectors are legalized and avoids processing the 4th component.

Original Patch by: arsenm (Matt Arsenault)

Differential Revision: https://reviews.llvm.org/D51589