RFR: JDK-8153930 - Compiler crashed (intermittently)

[Jim Laskey (Oracle)]

Two race conditions eliminated and ImageReader made idempotent.

http://cr.openjdk.java.net/~jlaskey/8153930/webrev/index.html <http://cr.openjdk.java.net/~jlaskey/8153930/webrev/index.html> (Note webrev fails to diff ImageReader.java, best to view new file.)
https://bugs.openjdk.java.net/browse/JDK-8153930 <https://bugs.openjdk.java.net/browse/JDK-8153930>