RFR: 8174050: Compilation errors with clang-4.0

[Martin Buchholz]

https://bugs.openjdk.java.net/browse/JDK-8174050
http://cr.openjdk.java.net/~martin/webrevs/openjdk10/pointer-sign-comparison/

My webrev could go in as is, but better would be for someone to figure out
the intent of the nonsensical assert in

src/share/vm/opto/loopPredicate.cpp

(There is more to be done to support clang, but this is enough to build
openjdk without patching source)