RFR: 8198523: Refactor BootstrapMethodInvoker to further avoid runtime type checks

[Claes Redestad]

Hi,

BootstrapMethodInvoker can be improved a bit further by providing
static information about the type argument, and additionally
teaching it about StringConcatFactory BSM:

Patch: http://cr.openjdk.java.net/~redestad/8198523/jdk.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8198523

This slightly improves startup on string concat tests.

Thanks!

/Claes