ICU-22991 Reduce unnecessary Grego calculation #76

Job	Run time
clang-valgrind-test	10m 41s
clang-valgrind-intltest (utility)	6m 58s
clang-valgrind-intltest (normalize)	7m 4s
clang-valgrind-intltest (collator)	5m 45s
clang-valgrind-intltest (regex)	5m 28s
clang-valgrind-intltest (format)	16m 53s
clang-valgrind-intltest (translit)	6m 7s
clang-valgrind-intltest (rbbi)	8m 18s
clang-valgrind-intltest (rbnf)	6m 50s
clang-valgrind-intltest (rbnfrt)	6m 21s
clang-valgrind-intltest (icuserv)	5m 41s
clang-valgrind-intltest (idna)	5m 44s
clang-valgrind-intltest (convert)	7m 8s
clang-valgrind-intltest (rbnfp)	5m 37s
clang-valgrind-intltest (csdet)	5m 45s
clang-valgrind-intltest (spoof)	5m 29s
clang-valgrind-intltest (bidi)	5m 54s
	2h 1m 43s

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