[Autobackout][BuildBreak]Revert of change: c538f3a

FlattenSmallSwitch: Ensure that all case blocks in the
switch have the switch block as their unique predecessor
to ensure that the switch block dominates the merge block.

Change-Id: I868a9bcc842bf6b4e233cb83686613fe26b531ce

Author: scottp101

IGC/Compiler/CustomSafeOptPass.cpp

@@ -3875,11 +3875,9 @@ IGC_INITIALIZE_PASS_BEGIN(GenStrengthReduction, "GenStrengthReduction",
     This class flatten small switch. For example,
 
     before optimization:
-        then153:
         switch i32 %115, label %else229 [
         i32 1, label %then214
         i32 2, label %then222
-        i32 3, label %then222 ; duplicate blocks are fine
         ]
 
         then214:                                          ; preds = %then153
@@ -3983,9 +3981,6 @@ bool FlattenSmallSwitch::processSwitchInst(SwitchInst* SI)
         if (br->getSuccessor(0) != MergeBlock)
             return false;
 
-        if (!BB->getUniquePredecessor())
-            return false;
-
         return true;
     };
 
@@ -4064,13 +4059,14 @@ bool FlattenSmallSwitch::processSwitchInst(SwitchInst* SI)
     // from BB to the InsertPoint.
     auto splice = [](BasicBlock* BB, Instruction* InsertPoint)
     {
-        for (auto II = BB->begin(), IE = BB->end(); II != IE; /* empty */)
+        Instruction* preIter = nullptr;
+        for (auto& iter : *BB)
         {
-            auto* I = &*II++;
-            if (I->isTerminator())
-                return;
-
-            I->moveBefore(InsertPoint);
+            if (preIter)
+            {
+                preIter->moveBefore(InsertPoint);
+            }
+            preIter = cast<Instruction>(&iter);
         }
     };
 
@@ -4109,8 +4105,6 @@ bool FlattenSmallSwitch::processSwitchInst(SwitchInst* SI)
     // connect the original block and the phi node block with a pass through branch
     builder.CreateBr(Dest);
 
-    SmallPtrSet<BasicBlock*, 4> Succs;
-
     // Remove the switch.
     BasicBlock* SelectBB = SI->getParent();
     for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i)
@@ -4120,9 +4114,7 @@ bool FlattenSmallSwitch::processSwitchInst(SwitchInst* SI)
         {
             continue;
         }
-
-        if (Succs.insert(Succ).second)
-            Succ->removePredecessor(SelectBB);
+        Succ->removePredecessor(SelectBB);
     }
     SI->eraseFromParent();