RFR: 8264271: Avoid creating non_oop_word oops

[Stefan Karlsson]

Some parts of the JVM puts an marker to show that a location does not contain a valid oop. The code that handles this typically look like this:

oop* p = ...
if (*p != Universe::non_oop_word())

This means that sometimes the *p will create an oop that contains the non_oop_word. This makes it problematic to add stricter oop verification. I propose that we add a new function that checks the value of locations without converting it to an oop. 

(Note: I'm testing the new dependent pull Skara feature with this PR. It builds depends on the pr/3214 branch)

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Depends on: https://git.openjdk.java.net/jdk/pull/3214

Commit messages:
 - 8264271: Avoid creating non_oop_word oops

Changes: https://git.openjdk.java.net/jdk/pull/3215/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3215&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264271
  Stats: 58 lines in 7 files changed: 38 ins; 10 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3215.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3215/head:pull/3215

PR: https://git.openjdk.java.net/jdk/pull/3215