RFR: 8071571: Move substring of same string to slow path

[Lev Priima]

Please review small cleanup in java.lang.String:

Issue: https://bugs.openjdk.java.net/browse/JDK-8071571
Webrev: http://cr.openjdk.java.net/~lpriima/8071571/0/webrev/

46 tests from jdk9/dev/jdk/test/java/lang/String* passed locally.

--
Lev