RFR: 8212726: Replace some use of drop- and foldArguments with filtering argument combinator in StringConcatFactory

[Claes Redestad]

Hi,

StringConcatFactory uses a customized internal foldArguments
implementation which takes positional arguments to avoid intermediary
permutation steps, see JDK-8165492[1]. My intent has been to clean this
up for possible inclusion in the public API.

In preparation of that, I realized that we could probably profit from a
filtering combinator on top of the existing folding one. Said and done:

http://cr.openjdk.java.net/~redestad/8212726/jdk.00/
https://bugs.openjdk.java.net/browse/JDK-8212726

Using this new MethodHandles.filterArgumentsWithCombiner in place of
MHs.dropArguments + MHs.foldArgumentsWithCombiner gets rid of a
significant number of MethodHandles with retained performance
characteristics. In startup tests exercising indified String
concatenation the number of generated classes drops by ~25-30%, with a
measurable improvement in startup time as a result. Subjectively it also
makes the code in StringConcatFactory easier to read and follow.

While this surely strengthens the case to include these in the public
API, I'd like to move ahead with this as an internal optimization first
and propose MethodHandles.filter-/foldArgumentsWithCombiner as public
API points as a follow-up: apart from improving the javadoc we need to
work out specification for corner cases - especially illegal values -
and harden the implementation with more tests. I'm sure there might be
opinions about the naming of these methods, too... :-)

Thanks!

/Claes

[1] https://bugs.openjdk.java.net/browse/JDK-8165492