Revert "[AMDGPU] Run `unreachable-mbb-elimination` after isel to clean up PHIs."

Revert "[AMDGPU] Run `unreachable-mbb-elimination` after isel to clean up PHIs."

Summary:
This has been superseded by "[AMDGPU]: PHI Elimination hooks added for custom COPY insertion."

This reverts the code changes from commit 53f967f2bdb6aa7b08596880c3689d1ecad6f0ff
but keeps the test case.

Reviewers: hliao, arsenm, tpr, dstuttard

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68769

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374347 91177308-0d34-0410-b5e6-96231b3b80d8