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This has been superseded by "[AMDGPU]: PHI Elimination hooks added for custom COPY insertion."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 53f967f2bd
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

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

@@ -885,9 +885,6 @@ bool GCNPassConfig::addInstSelector() {
   addPass(createSILowerI1CopiesPass());
   addPass(createSIFixupVectorISelPass());
   addPass(createSIAddIMGInitPass());
-  // FIXME: Remove this once the phi on CF_END is cleaned up by either removing
-  // LCSSA or other ways.
-  addPass(&UnreachableMachineBlockElimID);
   return false;
 }