[ImplicitNullChecks] NFC Fix the implicit-null-checks.mir test

Summary:
Currently the test implicit-null-checks.mir crashes if we run llc with
-enable-implicit-null-checks -start-before implicit-null-checks
options. Change fixes the RET instruction causing the crash.

Patch by Serguei Katkov!

Reviewers: sanjoy, reames

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29390

llvm-svn: 293789