[DAGCombine] Generalize Dead Store to overlapping stores.

Summary:
Remove stores that are immediately overwritten by larger
stores.

Reviewers: courbet, rnk

Reviewed By: rnk

Subscribers: javed.absar, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58467

llvm-svn: 354518