RFR: Fix Clang build warnings

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/build/macos-1/webrev.03/

These produce warnings for Clang, and thus break the build with Mac OS. Two cases were just
rewritten for clarity. The change in loopPredicate.cpp picks up the correct assert that we have
missed during the merge: http://hg.openjdk.java.net/shenandoah/jdk10/rev/1c9cefa2a443

Testing: hotspot_gc_shenandoah (Linux), build tests (by Lennart)

Thanks,
-Aleksey