[lworld] RFR: 8279624: [lworld] interpreter should avoid value copies in withfield when possible

[Frederic Parain]

Please review this small change to reduce the number of copies the interpreter does when executing the withfield bytecode.
If the new value of the field to be updated is identical to the old value of the field, the interpreter simply returns the original value instance instead of creating a new copy.
This change remove more than 8K value copies when running the hotspot_valhalla test suite.

Tested with Mach5, tiers 1 to 3.

Thank you

Fred

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Commit messages:
 - Avoid value copies in withfield when possible

Changes: https://git.openjdk.java.net/valhalla/pull/595/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=595&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279624
  Stats: 34 lines in 1 file changed: 33 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/595.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/595/head:pull/595

PR: https://git.openjdk.java.net/valhalla/pull/595