[ImplicitNullCheck] Extend canReorder scope

Summary:
This change allows a re-order of two intructions if their uses
are overlapped.

Patch by Serguei Katkov!

Reviewers: reames, sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29120

llvm-svn: 293775