RFR (8u): 8237837: Shenandoah: assert(mem == __null) failed: only one safepoint

[Roland Westrelin]

Initial change:
https://bugs.openjdk.java.net/browse/JDK-8237837
https://hg.openjdk.java.net/jdk/jdk/rev/a42cdcd11ca4

Backport:
http://cr.openjdk.java.net/~roland/8237837.8u/webrev.00/

The 8u change is identical to the upstream change except for code
related to loop strip mining that doesn't apply to 8u. I also had to
drop the @requires from the test case as they don't seem to work
properly on 8u. I verified the test fails without the patch and passes
with it. I also ran shenandoah tier1.

Roland.