[SelectionDAG] Replace the Chain in LOAD->VP_LOAD widening

The introduction of this legalization, D111248, forgot to replace the
old chain with the new. This could manifest itself in the old
(illegally-typed) value remaining in the DAG, though the simple test
cases didn't catch this.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D113561