RFR: 8158933: String concat stringifiers setup should avoid unnecessary lookups

[Claes Redestad]

Hi,

when generating code for a string concat, the MOST Function always does 
some lookup as well as generating a filtering method handle, but only 
use these values conditionally. This can also happen repeatedly when 
setting up concatenations with primitives (that aren't float or double).

By restructuring this code we can optimize the number of lookups made, 
slightly improving startup.

Webrev: http://cr.openjdk.java.net/~redestad/8158933/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8158933

Thanks!

/Claes