JEP 280 (Indify String Concat) integration

[Aleksey Shipilev]

Hi,

I think it is time to integrate JEP 280 ("Indify String Concat") into JDK 9:
 http://openjdk.java.net/jeps/280

The JEP is Targeted, the CCC is approved, the code reviews and
pre-integration checks come clean. In theory, we would like to integrate
with some "optimal" concat strategy switched by default, but we have to
wait for some Hotspot compiler changes to settle before we can make that
call. (Notably, https://bugs.openjdk.java.net/browse/JDK-8136469).

So, I would like to proceed in two parts:

 Part 1: Integrate all code changes, but keep the concatenation strategy
to be the same "naive" StringBuilder.append chains. In other words, we
are emitting string concat indy, but always link in the naive
StringBuilder.append code.

 Part 2: (After HS changes settle in) Re-run performance tests, and
switch to a more optimal concatenation strategy. This amounts to
one-liner change in a default value for Java property.

Thoughts? I would like relevant Project Leads / Reviewers (langtools and
jdk) to OKay the integration.

FTR, the webrevs:
  http://cr.openjdk.java.net/~shade/8085796/webrev.root.00/
  http://cr.openjdk.java.net/~shade/8085796/webrev.langtools.05/
  http://cr.openjdk.java.net/~shade/8085796/webrev.jdk.08/

Thanks,
-Aleksey