Accelerate java.lang.String.indexOf(String, int) in Java 9+

[fjeremic]

While investigating #6403 I noted that we did not have to make the fix for Java 9+. The reason is because of the way we implement the indexOf(String, int) API in Java 9+ [1] using the helpers already available in StringLatin1 and StringUTF16. These helper methods [2] [3] effectively reduce to the JITHelpers intrinsicIndexOfStringLatin1 and intrinsicIndexOfStringUTF16 APIs which already exist and which we already take advantage of for Java 8.

We need to investigate whether we can map the StringLatin1 and StringUTF16 intrinsics to our existing JITHelper methods and make sure we intrinsically reduce the java.lang.String.indexOf API on Java 9+. We should also take this opportunity to see if we can map the character overload of the indexOf API to the APIs which exist in StringLatin1 and StringUTF16.

[1] https://github.com/eclipse/openj9/blob/eb860ecf4551c9982b0bc4ca9c6fed61fe70868c/jcl/src/java.base/share/classes/java/lang/String.java#L2033-L2066
[2] https://github.com/ibmruntimes/openj9-openjdk-jdk11/blob/91be2043cf6c6008939b34e33ce381479638405c/src/java.base/share/classes/java/lang/StringLatin1.java#L232-L253
[3] https://github.com/ibmruntimes/openj9-openjdk-jdk11/blob/91be2043cf6c6008939b34e33ce381479638405c/src/java.base/share/classes/java/lang/StringUTF16.java#L383-L388