RFR: 8254807: Optimize startsWith() for String.substring() [v3]

[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 one additional commit since the last revision:

  use a brand new CallNode instead of overloading the startsWith.
  
  set_bci to -1 so we skip symbolic info validation

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/974/files
  - new: https://git.openjdk.java.net/jdk/pull/974/files/2543aa88..b8ef8ce7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=974&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=974&range=01-02

  Stats: 41 lines in 4 files changed: 10 ins; 9 del; 22 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