We can improve startup efficiency of the BootstrapMethodInvoker by adding back explicit casts to MethodType or Class as appropriate, and improve the structure of how we special case known BSMs.

Adding a special case for StringConcatFactory BSMs while we're at it can reduce bootstrap overhead of using indify string concat by a sizeable chunk (~200k executed bytecode).

PoC patch: http://cr.openjdk.java.net/~redestad/scratch/scf_bsm.patch