[DAGCombiner] don't use the pointer info for widen store

The merged store touches memory for other underlying objects, so mapping
the merged store to the first underlying object is not correct. For example
in https://github.com/llvm/llvm-project/issues/60744, the merged store is
not correctly analyzed as dependent with memory operations which are also
part of the merged store.

Fixes #60744

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D144711