[Assignment Tracking][SROA] Delete dbg.assigns linked to rewritten stores

AggLoadStoreRewriter splits aggregate loads and stores into scalars (before the
alloca is split up). The new stores and debug intrinsics are already wired up
correctly - we just need to also delete the dbg.assign that is linked to the
split to-be-deleted store too.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D142882