[13u] RFR: 8230565: ZGC: Redesign C2 load barrier to expand on the MachNode level

[Per Liden]

Hi,

Please review this back port of JDK-8230565 to 13u. This fix 
significantly improves stability of ZGC (or more specifically ZGC's C2 
load barrier). The back port was trivial, but not completely clean, as 
it required manual merge to resolve 4 simple conflicts.

Original bug: https://bugs.openjdk.java.net/browse/JDK-8230565
Original commit: https://hg.openjdk.java.net/jdk/jdk/rev/d376d86b0a01

JDK 13u webrev: http://cr.openjdk.java.net/~pliden/8230565/webrev.jdk13u

Testing:
- Tier1-3 on all Oracle platforms.
- Tier1-6 on Linux/x64.
- Multiple iterations of gc-test-suite on Linux/x64.
- Builds on Linux/AArch64.

/Per