RFR: 8254807: Optimize startsWith() for String.substring()

[Xin Liu]

the optimization transforms code from s=substring(base, beg, end); s.startsWith(prefix)
to substring(base, beg, end) | base.startsWith(prefix, beg). 

it reduces an use of substring. hopefully c2 optimizer can remove the used substring()

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Commit messages:
 - fix a regression test on x86_32
 - 8254807: Optimize startsWith() for String.substring()
 - 8254807: Optimize startsWith() for String.substring()
 - 8254807: Optimize startsWith() for String.substring()
 - 8254807: Optimize startsWith() for String.substring()
 - 8254807: Optimize startsWith() for String.substring()
 - 8254807: Optimize startsWith() for String.substring()

Changes: https://git.openjdk.java.net/jdk/pull/974/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=974&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254807
  Stats: 538 lines in 15 files changed: 472 ins; 56 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/974.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/974/head:pull/974

PR: https://git.openjdk.java.net/jdk/pull/974