Bitcode: Don't optimize constants when preserving use-list order

`ValueEnumerator::OptimizeConstants()` creates forward references within
the constant pools, which makes predicting constants' use-list order
difficult.  For now, just disable the optimization.

This can be re-enabled in the future in one of two ways:

  - Enable a limited version of this optimization that doesn't create
    forward references.  One idea is to categorize constants by their
    "height" and make that the top-level sort.

  - Enable it entirely.  This requires predicting how may times each
    constant will be recreated as its operands' and operands' operands'
    (etc.) forward references get resolved.

This is part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213953 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Writer/ValueEnumerator.cpp

@@ -18,6 +18,7 @@
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/UseListOrder.h"
 #include "llvm/IR/ValueSymbolTable.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
@@ -179,6 +180,11 @@ void ValueEnumerator::print(raw_ostream &OS, const ValueMapType &Map,
 void ValueEnumerator::OptimizeConstants(unsigned CstStart, unsigned CstEnd) {
   if (CstStart == CstEnd || CstStart+1 == CstEnd) return;
 
+  if (shouldPreserveBitcodeUseListOrder())
+    // Optimizing constants makes the use-list order difficult to predict.
+    // Disable it for now when trying to preserve the order.
+    return;
+
   std::stable_sort(Values.begin() + CstStart, Values.begin() + CstEnd,
                    [this](const std::pair<const Value *, unsigned> &LHS,
                           const std::pair<const Value *, unsigned> &RHS) {