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This has been superseded by "[AMDGPU]: PHI Elimination hooks added for custom COPY insertion."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 53f967f2
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

llvm-svn: 374347