IPO: Add use-list-order verifier

Add a -verify-use-list-order pass, which shuffles use-list order, writes
to bitcode, reads back, and verifies that the (shuffled) order matches.

  - The utility functions live in lib/IR/UseListOrder.cpp.

  - Moved (and renamed) the command-line option to enable writing
    use-lists, so that this pass can return early if the use-list orders
    aren't being serialized.

It's not clear that this pass is the right direction long-term (perhaps
a separate tool instead?), but short-term it's a great way to test the
use-list order prototype.  I've added an XFAIL-ed testcase that I'm
hoping to get working pretty quickly.

This is part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213945 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/UseListOrder.h

@@ -0,0 +1,42 @@
+//===- llvm/IR/UseListOrder.h - LLVM Use List Order functions ---*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file has functions to modify the use-list order and to verify that it
+// doesn't change after serialization.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_IR_USELISTORDER_H
+#define LLVM_IR_USELISTORDER_H
+
+namespace llvm {
+
+class Module;
+
+/// \brief Whether to preserve use-list ordering.
+bool shouldPreserveBitcodeUseListOrder();
+
+/// \brief Shuffle all use-lists in a module.
+///
+/// Adds \c SeedOffset to the default seed for the random number generator.
+void shuffleUseLists(Module &M, unsigned SeedOffset = 0);
+
+/// \brief Verify use-list order after serializing to bitcode.
+///
+/// \return \c true if there are no errors.
+bool verifyBitcodeUseListOrder(const Module &M);
+
+/// \brief Verify use-list order after serializing to assembly.
+///
+/// \return \c true if there are no errors.
+bool verifyAssemblyUseListOrder(const Module &M);
+
+} // end namespace llvm
+
+#endif