RFR: 8254807: Optimize startsWith() for String.substring() [v7]

[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 uses of substring. hopefully c2 optimizer can remove the used substring.

Xin Liu has updated the pull request incrementally with two additional commits since the last revision:

 - fix crash when users disable inliner using -XX:-Inline
   
   when inliner is off, substring optimization is skipped as well
   because it depends on late_inliner.
 - do api transformation for trivial substrings.
   
   A substring is trivial if it is generated from String.substring(base, beginIndex)
   String.substring(base, beginIndex, endIndex) is not trivial unless endIndex == base.length().
   
   This patch fixes the wrong transformation like this because x isn't trivial here.
   String s = "abcd"; String x = s.substring(1, 2); x.startsWith("bc");

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/974/files
  - new: https://git.openjdk.java.net/jdk/pull/974/files/c5a25bfb..044fdb41

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=974&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=974&range=05-06

  Stats: 83 lines in 4 files changed: 73 ins; 4 del; 6 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