RFR (S) 8150463: StringConcat MH_INLINE_SIZED_EXACT should skip storage initialization

[Aleksey Shipilev]

Hi,

I'd like to improve our advanced String concat strategy,
MH_INLINE_SIZED_EXACT, with skipping the storage initialization:
  https://bugs.openjdk.java.net/browse/JDK-8150463

Webrev:
  http://cr.openjdk.java.net/~shade/8150463/webrev.01/

Performance had improved, as planned:
  http://cr.openjdk.java.net/~shade/8150463/perf.txt

The rest of the difference against BC_SB strategy is code-generation
quirks, to be addressed separately. We pay some price for a runtime
final index check to strengthen against inadvertent bugs in
concatenation code.

This patch does not affect the String concatenation at this point,
because we are still running with BC_SB strategy. I ran through
java/lang/String jtregs to see if MH_INLINE_SIZED_EXACT works as
expected. Therefore, I don't think this needs to wait for Jigsaw
integration. But, it can also wait, no problem. I'll let Jigsaw folks to
decide on integration timing.

Thanks,
-Aleksey