[13] RFR (S): 8217358: Optimized build is broken by Shenandoah changes

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8217358/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8217358

There's an inconsistency in how assert helpers are declared in 
Shenandoah code: PRODUCT_RETURN is used in headers, but implementations 
are guarded by ASSERT.

Proposed fix uses NOT_DEBUG_RETURN instead.

Best regards,
Vladimir Ivanov