[TASK] Optimize GeneralUtility path methods

The following methods are amongst the most frequently called ones:
* getFileAbsFileName()
* validPathStr()
* isAllowedAbsPath()
* verifyFilenameAgainstDenyPattern()

They can be optimized to return early, save some variable assignments and
save some preg_match calls.

Change-Id: Id30b2f9b5a053d4267d9c24339f414821ba661ea
Resolves: #54525
Releases: 6.2
Reviewed-on: https://review.typo3.org/26505
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Stefan Neufeind
Reviewed-by: Markus Klein
Tested-by: Markus Klein