[9] RFR(S): 8059331: Print additional information for the assert in Compile::start()

[Albert Noll]

Hi,

could I get reviews for this small patch?

Bug: https://bugs.openjdk.java.net/browse/JDK-8059331
Webrev: http://cr.openjdk.java.net/~anoll/8059529/webrev.00/

The assert failed once in JDK-8058434. Printing the failure reason in 
the assert helps
tracking down the problem.

Best,
Albert