[11u] RFR 8235762: JVM crash in SWPointer during C2 compilation

[Roland Westrelin]

Original bug:
https://bugs.openjdk.java.net/browse/JDK-8235762
https://hg.openjdk.java.net/jdk/jdk/rev/5aa85653a589

Original patch does not apply cleanly to 11u in
SuperWord::find_adjacent_refs(). The original change in jdk 14 partially
reverts a previous change in SuperWord::find_adjacent_refs() that's not
in jdk 11. With the patch I propose, SuperWord::find_adjacent_refs() is
mostly identical to today's code in jdk 15.

11u webrev:
  http://cr.openjdk.java.net/~roland/8235762.11u/webrev.00/

Testing: x86_64/aarch4 build, verified new test fails without the patch
and runs fine with it on aarch64, tier1 + tier2

Roland.