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/Transforms/IPO.h

@@ -118,6 +118,11 @@ ModulePass *createInternalizePass(ArrayRef<const char *> ExportList);
 /// createInternalizePass - Same as above, but with an empty exportList.
 ModulePass *createInternalizePass();
 
+/// \brief Verify that use-list order doesn't change after shuffling.
+///
+/// \note This is a transformation, since the use-list order changes.
+ModulePass *createVerifyUseListOrderPass();
+
 //===----------------------------------------------------------------------===//
 /// createDeadArgEliminationPass - This pass removes arguments from functions
 /// which are not used by the body of the function.