[libyul] Fix false-negative maybe-uninitialized error on gcc.

[aarlt]

Building with gcc v13.3.0 was not possible due to maybe-uninitialized errors, e.g.

In file included from /usr/include/c++/13/bits/shared_ptr.h:53,
                 from /usr/include/c++/13/memory:80,
                 from /home/parallels/git/http/solidity/liblangutil/SourceLocation.h:27,
                 from /home/parallels/git/http/solidity/libsolutil/Exceptions.h:21,
                 from /home/parallels/git/http/solidity/libyul/Exceptions.h:24,
                 from /home/parallels/git/http/solidity/libyul/optimiser/ASTWalker.h:26,
                 from /home/parallels/git/http/solidity/libyul/optimiser/ForLoopConditionIntoBody.h:20,
                 from /home/parallels/git/http/solidity/libyul/optimiser/ForLoopConditionIntoBody.cpp:19:
In destructor ‘std::__shared_count<_Lp>::~__shared_count() [with __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’,
    inlined from ‘std::__shared_ptr<_Tp, _Lp>::~__shared_ptr() [with _Tp = const solidity::langutil::DebugData; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]’ at /usr/include/c++/13/bits/shared_ptr_base.h:1524:7,
    inlined from ‘std::shared_ptr<const solidity::langutil::DebugData>::~shared_ptr()’ at /usr/include/c++/13/bits/shared_ptr.h:175:11,
    inlined from ‘virtual void solidity::yul::ForLoopConditionIntoBody::operator()(solidity::yul::ForLoop&)’ at /home/parallels/git/http/solidity/libyul/optimiser/ForLoopConditionIntoBody.cpp:50:7:
/usr/include/c++/13/bits/shared_ptr_base.h:1070:13: error: ‘<unnamed>.solidity::yul::BuiltinName::debugData.std::shared_ptr<const solidity::langutil::DebugData>::<unnamed>.std::__shared_ptr<const solidity::langutil::DebugData, __gnu_cxx::_S_atomic>::_M_refcount.std::__shared_count<>::_M_pi’ may be used uninitialized [-Werror=maybe-uninitialized]
 1070 |         if (_M_pi != nullptr)
      |             ^~~~~
/home/parallels/git/http/solidity/libyul/optimiser/ForLoopConditionIntoBody.cpp: In member function ‘virtual void solidity::yul::ForLoopConditionIntoBody::operator()(solidity::yul::ForLoop&)’:
/home/parallels/git/http/solidity/libyul/optimiser/ForLoopConditionIntoBody.cpp:50:114: note: ‘<anonymous>’ declared here
   50 |                                                 BuiltinName{debugData, *m_dialect.booleanNegationFunctionHandle()},
      |                                                                                                                  ^
cc1plus: all warnings being treated as errors
make[2]: *** [libyul/CMakeFiles/yul.dir/build.make:860: libyul/CMakeFiles/yul.dir/optimiser/ForLoopConditionIntoBody.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:562: libyul/CMakeFiles/yul.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

[cameel]

Looks fine, but things like for things like this you should always include some context. I.e. add a comment saying why you're doing it and maybe paste the relevant bits of the discussion into the PR description. If I hadn't seen the discussion, I'd have no idea whether this change makes sense or not.