Optimize `String#rchop?()` #3509

Job	Run time
check_format	1m 32s
x86_64-gnu-test (1.7.3)	1s
x86_64-gnu-test (1.8.2)	1s
x86_64-gnu-test (1.9.2)	1s
x86_64-gnu-test (1.10.1)	11m 26s
x86_64-gnu-test (1.11.2)	8m 14s
x86_64-gnu-test (1.12.2)	2m 52s
x86_64-gnu-test (1.13.3)	8m 21s
x86_64-gnu-test (1.14.0)	2m 48s
x86_64-gnu-test-preview_mt	5m 33s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	11m 26s
x86_64-musl-test	11m 47s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	11m 30s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	2m 15s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	1s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	1s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	1s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	1s
	1h 17m 51s

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