[SIL Utils] Move IndexTrieNode into its own header in Utils. NFC.

Move IndexTrieNode from DeadObjectElimination into its own header. I plan to
use this data structure when diagnosing static violations of exclusive access.

include/swift/SILOptimizer/Utils/IndexTrie.h

@@ -0,0 +1,66 @@
+//===--- IndexTrie - Trie for a sequence of integer indices ----*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_SILOPTIMIZER_UTILS_INDEXTREE_H
+#define SWIFT_SILOPTIMIZER_UTILS_INDEXTREE_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SmallVector.h"
+#include <algorithm>
+
+namespace swift {
+
+// Trie node representing a sequence of unsigned integer indices.
+class IndexTrieNode {
+  static const unsigned RootIdx = ~0U;
+  unsigned Index;
+  llvm::SmallVector<IndexTrieNode*, 8> Children;
+
+public:
+  IndexTrieNode(): Index(RootIdx) {}
+
+  explicit IndexTrieNode(unsigned V): Index(V) {}
+
+  IndexTrieNode(IndexTrieNode &) =delete;
+  IndexTrieNode &operator=(const IndexTrieNode&) =delete;
+
+  ~IndexTrieNode() {
+    for (auto *N : Children)
+      delete N;
+  }
+
+  bool isRoot() const { return Index == RootIdx; }
+
+  bool isLeaf() const { return Children.empty(); }
+
+  unsigned getIndex() const { return Index; }
+
+  IndexTrieNode *getChild(unsigned Idx) {
+    assert(Idx != RootIdx);
+
+    auto I = std::lower_bound(Children.begin(), Children.end(), Idx,
+                              [](IndexTrieNode *a, unsigned i) {
+                                return a->Index < i;
+                              });
+    if (I != Children.end() && (*I)->Index == Idx)
+      return *I;
+    auto *N = new IndexTrieNode(Idx);
+    Children.insert(I, N);
+    return N;
+  }
+
+  ArrayRef<IndexTrieNode*> getChildren() const { return Children; }
+};
+
+} // end namespace swift
+
+#endif