OptimizeStringConcat has an opaque dependency on Integer.sizeTable field

During the work on JDK-8136500, it was discovered that OptimizeStringConcat code in C2 has its own Integer.stringSize copy, which references Integer.sizeTable static field. Removing that static field silently breaks OptimizeStringConcat, making it to bail every time. (There is an assert that fails in fastdebug build).

After JDK-8136500 is done, we need to figure out the way to remove Integer.sizeTable and/or rewrite the PhaseStringOpts::int_stringSize to match JDK-8136500 changes.