RFR: add use site null checks for fields

[Vicente Romero]

generate null checks for field use sites depending on a compiler option, see also [1] which did a similar thing for method invocations

[1] https://github.com/openjdk/valhalla/pull/1910

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Commit messages:
 - minor fix
 - add use site null checks for fields

Changes: https://git.openjdk.org/valhalla/pull/1926/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1926&range=00
  Stats: 42 lines in 3 files changed: 38 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/valhalla/pull/1926.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1926/head:pull/1926

PR: https://git.openjdk.org/valhalla/pull/1926