[ImplicitNullCheck] Extend Implicit Null Check scope by using stores

Summary:
This change allows usage of store instruction for implicit null check.

Memory Aliasing Analisys is not used and change conservatively supposes
that any store and load may access the same memory. As a result
re-ordering of store-store, store-load and load-store is prohibited.

Patch by Serguei Katkov!

Reviewers: reames, sanjoy

Reviewed By: sanjoy

Subscribers: atrick, llvm-commits

Differential Revision: https://reviews.llvm.org/D29400

llvm-svn: 294338