C1: Raw version of UnsafeGet generates load barriers

After rebasing the ZGC Generational code to latest openjdk/jdk we hit a crash because a load barrier was generated for some oops that are not supposed to have a load barrier. In upstream ZGC this isn't a problem because these load barriers are just redundant and will find healed good oops. However, in the Generational version un-barriered roots (e.g. thread stacks) have no metadata bits, and we should never perform normal load barriers on them (they are handled by stack watermarks and nmethod barriers).

This load barrier was introduced with:
JDK-8266746: C1: Replace UnsafeGetRaw with UnsafeGet when setting up OSR entry block

which makes UnsafeGet _also_ handle "raw" loads. However, the "raw" path introduced a load barrier. Note that "raw" is often used to denote a function that loads/stores values without using a barrier.