RenameFunctionRector: prevent repeated isName() calls

[staabm]

because it showed up in codeigniter profiles

[staabm]

tested on codeigniter4 codebase:

before this PR:

time php vendor/bin/rector process tests --dry-run --clear-cache
 326/326 [============================] 100%

 [OK] Rector is done!

real    0m58.909s
user    0m0.000s
sys     0m0.031s

after this PR

 time php vendor/bin/rector process tests --dry-run --clear-cache
 326/326 [============================] 100%

 [OK] Rector is done!

real    0m54.508s
user    0m0.031s
sys     0m0.015s

=> 7-8% faster

[samsonasik]

Thank you @staabm

[staabm]

interessting side-effect: it seems because we do less isName() calls, we also do less reflection stuff which - in case we reflect on a identifier which does not exit - consumes more memory: (on the codeigniter4 codebase)