Faster MutatingScope->getNodeKey()

[staabm]

we call getNodeKey() several million times in most profiles

after

➜  phpstan-src git:(2.0.x) ✗ hyperfine 'php bin/phpstan analyze --debug src/Rules' -i
Benchmark 1: php bin/phpstan analyze --debug src/Rules
  Time (mean ± σ):     14.899 s ±  0.110 s    [User: 14.583 s, System: 0.310 s]
  Range (min … max):   14.667 s … 15.006 s    10 runs

before

➜  phpstan-src git:(2.0.x) ✗ hyperfine 'php bin/phpstan analyze --debug src/Rules' -i
Benchmark 1: php bin/phpstan analyze --debug src/Rules
  Time (mean ± σ):     14.980 s ±  0.148 s    [User: 14.645 s, System: 0.316 s]
  Range (min … max):   14.844 s … 15.332 s    10 runs

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

This improvement is really negligible, but I don't see a reason against merging this. Thakns.