Optimize `String#rchop?()` #3510

Job	Run time
check_format	1m 37s
x86_64-gnu-test-preview_mt	5m 41s
x86_64-gnu-test (1.7.3)	33m 56s
x86_64-gnu-test (1.8.2)	33m 15s
x86_64-musl-test	36m 38s
x86_64-gnu-test (1.9.2)	33m 41s
x86_64-gnu-test (1.10.1)	35m 4s
x86_64-gnu-test (1.11.2)	34m 40s
x86_64-gnu-test (1.12.2)	33m 37s
x86_64-gnu-test (1.13.3)	34m 53s
x86_64-gnu-test (1.14.0)	37m 22s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 11s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 20s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	34m 34s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	33m 13s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	32m 54s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 59s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	35m 30s
	9h 18m 5s

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