AMDGPU: Use eraseFromParent to delete am instruction when it is no longer needed.

[cfang]

Summary

We should delete the instruction when it is not needed any more.
Done in file AMDGPUUnifyDivergentExitNodes.cpp

llvm/trunk/lib/Target/AMDGPU/AMDGPUUnifyDivergentExitNodes.cpp

static BasicBlock *unifyReturnBlockSet(Function &F,
   // Loop over all of the blocks, replacing the return instruction with an
   // unconditional branch.
   for (BasicBlock *BB : ReturningBlocks) {
     // Add an incoming element to the PHI node for every return instruction that
     // is merging into this new block...
     if (PN)
       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
 
-    BB->getInstList().pop_back();  // Remove the return insn
+    // Remove and delete the return inst.
+    BB->getTerminator()->eraseFromParent();
     BranchInst::Create(NewRetBlock, BB);
   }
 
   for (BasicBlock *BB : ReturningBlocks) {
     // Cleanup possible branch to unconditional branch to the return.
     simplifyCFG(BB, TTI, {2});
   }
 
if (!UnreachableBlocks.empty()) {
     if (UnreachableBlocks.size() == 1) {
       UnreachableBlock = UnreachableBlocks.front();
     } else {
       UnreachableBlock = BasicBlock::Create(F.getContext(),
                                             "UnifiedUnreachableBlock", &F);
       new UnreachableInst(F.getContext(), UnreachableBlock);
 
       for (BasicBlock *BB : UnreachableBlocks) {
-        BB->getInstList().pop_back();  // Remove the unreachable inst.
+        // Remove and delete the unreachable inst.
+        BB->getTerminator()->eraseFromParent();
         BranchInst::Create(UnreachableBlock, BB);
       }
     }
 
     if (!ReturningBlocks.empty()) {
       // Don't create a new unreachable inst if we have a return. The
       // structurizer/annotator can't handle the multiple exits
 
       Type *RetTy = F.getReturnType();
       Value *RetVal = RetTy->isVoidTy() ? nullptr : UndefValue::get(RetTy);
-      UnreachableBlock->getInstList().pop_back();  // Remove the unreachable inst.
+      // Remove and delete the unreachable inst.
+      UnreachableBlock->getTerminator()->eraseFromParent();
 
       Function *UnreachableIntrin =
         Intrinsic::getDeclaration(F.getParent(), Intrinsic::amdgcn_unreachable);
 
       // Insert a call to an intrinsic tracking that this is an unreachable
       // point, in case we want to kill the active lanes or something later.
       CallInst::Create(UnreachableIntrin, {}, "", UnreachableBlock);