[lworld] RFR: 8282512: Remove usages of __WithField in compiler tests

[Dan Smith]

Rewrote 2 compiler tests to perform 'withfield' operations in jasm code rather than using the '__WithField' javac hack.

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Commit messages:
 - Remove -XDallowWithFieldOperator usages
 - Fixes for TestWithfieldC1
 - Add TestDeoptimizationWhenBuffering changes
 - Fix TestDeoptimizationWhenBuffering

Changes: https://git.openjdk.java.net/valhalla/pull/664/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=664&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282512
  Stats: 458 lines in 5 files changed: 293 ins; 160 del; 5 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/664.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/664/head:pull/664

PR: https://git.openjdk.java.net/valhalla/pull/664