RFR: 8198888: Reduce string allocation churn in InvokerBytecodeGenerator

[Claes Redestad]

Hi,

two trivial optimizations that get rid of a few percent on ISC startup 
tests.

Webrev: http://cr.openjdk.java.net/~redestad/8198888/jdk.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8198888

Thanks!

/Claes