SIL: Replace TransitivelyUnreachableBlocks with DeadEndBlocks

We had both utilities doing the same thing.
NFC

lib/SIL/BasicBlockUtils.cpp

@@ -0,0 +1,41 @@
+//===--- BasicBlockUtils.cpp - Utilities for SILBasicBlock ----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/SIL/BasicBlockUtils.h"
+#include "swift/SIL/SILFunction.h"
+#include "swift/SIL/SILBasicBlock.h"
+
+using namespace swift;
+
+//===----------------------------------------------------------------------===//
+//                              DeadEndBlocks
+//===----------------------------------------------------------------------===//
+
+void DeadEndBlocks::compute() {
+  assert(ReachableBlocks.empty() && "Computed twice");
+
+  // First step: find blocks which end up in a no-return block (terminated by
+  // an unreachable instruction).
+  // Search for function-exiting blocks, i.e. return and throw.
+  for (const SILBasicBlock &BB : *F) {
+    const TermInst *TI = BB.getTerminator();
+    if (TI->isFunctionExiting())
+      ReachableBlocks.insert(&BB);
+  }
+  // Propagate the reachability up the control flow graph.
+  unsigned Idx = 0;
+  while (Idx < ReachableBlocks.size()) {
+    const SILBasicBlock *BB = ReachableBlocks[Idx++];
+    for (SILBasicBlock *Pred : BB->getPredecessorBlocks())
+      ReachableBlocks.insert(Pred);
+  }
+}