Speed up `IsRegularPGroup` #5720

Job	Run time
Test in OpenBSD	8m 5s
Cross compiler for s390x	28m 5s
testinstall - - ubuntu-latest	4m 24s
teststandard - - ubuntu-latest	18m 54s
testmanuals - - ubuntu-latest	5m 41s
testexpect testmockpkg testspecial test-compile testlibgap testkernel testworkspace - - ubuntu-latest	6m 20s
testpackages testinstall-loadall - - ubuntu-latest	26m 28s
makemanuals testmakeinstall - CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	5m 19s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	18m 13s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	7m 25s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	3m 17s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	27m 59s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	4m 28s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 2s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	4m 35s
testmockpkg testinstall - - windows-2019	14m 49s
Send Slack notification on status change	0s
	3h 8m 4s

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