Revert "Recommit "[ConstraintElimination] Change debug output to display variable names.""

This reverts commit 2a2a6bfc.

This causes build failures:

    /home/npopov/repos/llvm-project/llvm/lib/Analysis/ConstraintSystem.cpp: In member function ‘llvm::SmallVector<std::__cxx11::basic_string<char> > llvm::ConstraintSystem::getVarNamesList() const’:
    /home/npopov/repos/llvm-project/llvm/lib/Analysis/ConstraintSystem.cpp:118:10: error: invalid use of incomplete type ‘class llvm::Value’
      118 |     if (V->getName().empty())
          |          ^~
    In file included from /home/npopov/repos/llvm-project/llvm/lib/Analysis/ConstraintSystem.cpp:9:
    /home/npopov/repos/llvm-project/llvm/include/llvm/Analysis/ConstraintSystem.h:21:7: note: forward declaration of ‘class llvm::Value’
       21 | class Value;
          |       ^~~~~
    /home/npopov/repos/llvm-project/llvm/lib/Analysis/ConstraintSystem.cpp:119:22: error: invalid use of incomplete type ‘class llvm::Value’
      119 |       OperandName = V->getNameOrAsOperand();
          |                      ^~
    /home/npopov/repos/llvm-project/llvm/include/llvm/Analysis/ConstraintSystem.h:21:7: note: forward declaration of ‘class llvm::Value’
       21 | class Value;
          |       ^~~~~
    /home/npopov/repos/llvm-project/llvm/lib/Analysis/ConstraintSystem.cpp:121:41: error: invalid use of incomplete type ‘class llvm::Value’
      121 |       OperandName = std::string("%") + V->getName().str();
          |                                         ^~
    /home/npopov/repos/llvm-project/llvm/include/llvm/Analysis/ConstraintSystem.h:21:7: note: forward declaration of ‘class llvm::Value’
       21 | class Value;
          |       ^~~~~