Revert "[NFC][ScheduleDAG] Remove unused EntrySU SUnit"

This reverts commit 0345d88de654259ae90494bf9b015416e2cccacb.

Google internal backend uses EntrySU, we are looking into removing
dependency on it.

Differential Revision: https://reviews.llvm.org/D88018
diff --git a/llvm/lib/Target/AMDGPU/GCNMinRegStrategy.cpp b/llvm/lib/Target/AMDGPU/GCNMinRegStrategy.cpp
index 417abc4..884b2e1 100644
--- a/llvm/lib/Target/AMDGPU/GCNMinRegStrategy.cpp
+++ b/llvm/lib/Target/AMDGPU/GCNMinRegStrategy.cpp
@@ -248,6 +248,7 @@
   for (auto SU : TopRoots) {
     RQ.push_back(*new (Alloc.Allocate()) Candidate(SU, StepNo));
   }
+  releaseSuccessors(&DAG.EntrySU, StepNo);
 
   while (!RQ.empty()) {
     LLVM_DEBUG(dbgs() << "\n=== Picking candidate, Step = " << StepNo