[AMDGPU] Fix AMDGPUUnifyDivergentExitNodes

Summary:
For the case where "done" bits on existing exports are removed
by unifyReturnBlockSet(), unify all return blocks - even the
uniformly reached ones. We do not want to end up with a non-unified,
uniformly reached block containing a normal export with the "done"
bit cleared.

That case is believed to be rare - possible with infinite loops
in pixel shaders.

This is a fix for D71192.

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76364