message: fix affine evaluation for constraints with constant terms
diff: @@ -821,10 +821,15 @@ llvm::Value* Compiler::ElementPtr(const buffer& buf) {
  llvm::Value* Compiler::Eval(const stripe::Affine& access) {
  llvm::Value* offset = IndexConst(0);
  for (auto& term : access.getMap()) {
  + llvm::Value* indexVal = nullptr;
  + if (!term.first.empty()) {
  llvm::Value* indexVar = indexes_[term.first].variable;
  - llvm::Value* indexVal = builder_.CreateLoad(indexVar);
  + indexVal = builder_.CreateLoad(indexVar);
  llvm::Value* multiplier = IndexConst(term.second);
  indexVal = builder_.CreateMul(indexVal, multiplier);
  + } else {
  + indexVal = IndexConst(term.second);
  + }
  offset = builder_.CreateAdd(offset, indexVal);
  }
  return offset;
  