[CLEANUP] Simplify method structures in IRRE JS

There were some deeply nested if-structures in the IRRE JS code, which
are replaced with guard clauses with early returns in this commit.

Change-Id: I8d16b1860679b96ce20a7f484bcccb5a1d395091
Related: #58475
Releases: master
Reviewed-on: http://review.typo3.org/29836
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>