RFR: 8245959: Extend String concat testing to account for folded constants

[Claes Redestad]

Hi,

Since JDK-8222852 the default String concat strategy aggressively folds
string constants into prepender method handles, which means an
expression like foo + bar and one like "x" + foo + "y" + bar + "z" will
have similar shape, but take slightly different paths through generated
code.

This patch extends the ImplicitStringConcatShapes test to add more
shape variants with surrounding String constants, which should improve
coverage and put a bit more stress on the implementation strategy.

The changes of interest are in ImplicitStringConcatShapesTestGen -
since that's what is used to generate ImplicitStringConcatShapes

Bug:    https://bugs.openjdk.java.net/browse/JDK-8245959
Webrev: http://cr.openjdk.java.net/~redestad/8245959/open.00/

Testing: tier1

Thanks!

/Claes