RFR: 8222484: Specialize generation of simple String concatenation expressions

[Claes Redestad]

Hi,

please review this enhancement to specialize a few simple and common
String concatenation patterns in the default strategy.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8222484
Webrev: http://cr.openjdk.java.net/~redestad/8222484/open.00/

This reduces first-time bootstrap times for the targeted patterns from
~20ms to be lost in the noise on my machine (so ~0.5ms), while being
peak performance neutral and not regressing bootstrap characteristics on
more complex patterns.

This also consolidates the String::concat method to reuse the optimized
method in StringConcatHelper, which speeds up that method by ~1.4x for a
few sampled inputs.

Testing: tier1-3

Thanks!

/Claes