lgtm: detect more possible problematic scenarios

lgtm: detect more possible problematic scenarios

1) don't ignore stack-allocated variables, since they may hide
   heap-allocated stuff (compound types)
2) check if there's a return between the variable declaration and its
   initialization; if so, treat the variable as uninitialized
3) introduction of 2) increased the query runtime exponentially, so
   introduce some optimizations to bring it back to some reasonable
   values