RFR(XS): 8073954: Initialize _replaced_nodes in SafePoints.

[Lindenmaier, Goetz]

Hi,

please review this tiny fix in the C2 compiler.  I please need a sponsor.

The field _replaced_node added to SafePoints in "8026796: Make replace_in_map() on parent maps generic"
is not initialized to NULL.  As the memory is pre-set to 0, this doesn't cause immediate problems.
But we saw lots of sporadic failures in our nightly tests of SAP JVM with the debug build, where memory
in Arenas is zapped to 0xab.  We also saw rare failures with the opt builds.

https://bugs.openjdk.java.net/browse/JDK-8073954
http://cr.openjdk.java.net/~goetz/webrevs/8073954-replNd/webrev/

8026796 was downported to 8u40, so it's wrong there, too.

Best regards,
  Goetz.