hg: jdk9/sandbox/langtools: Only use IndyStringConcat for JDK >= 9

[aleksey.shipilev at oracle.com]

Changeset: cce61273dbe6
Author:    shade
Date:      2015-11-13 02:06 +0300
URL:       http://hg.openjdk.java.net/jdk9/sandbox/langtools/rev/cce61273dbe6

Only use IndyStringConcat for JDK >= 9

! src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/Gen.java
! src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/Target.java
+ test/tools/javac/TestIndyStringConcat.java