RFR: 8223454: Reduce String concatenation shapes by folding initialLengthCoder into last mixer

[Claes Redestad]

Hi,

yet another String concat startup/footprint optimization.

As the subject implies, we could fold the initialLengthCoder it into the
final mixer. This way we end up with one less bound argument into the
larger method handle combinator tree, with fewer species classes and
lambda forms generated as a result. This means a significant speed-up on
several startup and footprint tests, and especially noticeable on
realistic workloads that just have a few different small concat shapes.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8223454
Webrev: http://cr.openjdk.java.net/~redestad/8223454/open.00/

Thanks!

/Claes