AMDGPU: Re-do update for branch-relaxation test

Modify the test so that it is still testing something
closer to what it was intended to originally.

I think the original intent was to test the situation where
there was a branch on execz and then unconditional branch
required relaxing.With the change in r296539,
there was no longer and execz branch.

Change the test so that there is now an execz branch inserted.
There is no longer an unconditional branch after the execz branch,
so this might need to be tricked in some other way to keep that
there.

llvm-svn: 296574