similar variables detector is extremely slow

This detector makes up the majority of runtime on large codebases
```
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.000 0.000 41.286 41.286 /Users/alpharush/tob/slither/slither/__main__.py:81(process_all)
1 0.000 0.000 39.059 39.059 /Users/alpharush/tob/slither/slither/__main__.py:58(process_single)
1 0.000 0.000 33.319 33.319 /Users/alpharush/tob/slither/slither/__main__.py:111(_process)
1 0.000 0.000 33.317 33.317 /Users/alpharush/tob/slither/slither/slither.py:198(run_detectors)
1 0.000 0.000 33.317 33.317 /Users/alpharush/tob/slither/slither/slither.py:204()
84 0.001 0.000 33.317 0.397 /Users/alpharush/tob/slither/slither/detectors/abstract_detector.py:176(detect)
1 0.000 0.000 31.215 31.215 /Users/alpharush/tob/slither/slither/detectors/variables/similar_variables.py:72(_detect)

```
https://github.com/crytic/slither/blob/master/slither/detectors/variables/similar_variables.py#L63-L66