Do not use ArrayList when LambdaForm has a single ClassData

The ClassData of a LambdaForm class is always passed as an ArrayList, even if the list contains a single element

https://github.com/openjdk/jdk/blob/21407dec0156301871a83328615e4d975c4287c4/src/java.base/share/classes/java/lang/invoke/InvokerBytecodeGenerator.java#L382-L394

We should optimize for the single-element case for a small code size and speed improvement.

Data with the exploded JDK:

$ java -cp . HelloWorld

total generated LambdaForms: 34
with a single ClassData: 34

$ java -jar $ECLIPSE

total generated LambdaForms: 427
with a single ClassData: 406

(There are no LambdaForms with 0 ClassData in either case, so that's not worth optimizing).