RFR: add use site null checks

[Vicente Romero]

Adding use site null checks for method invocations. They are added by default but if option: `noUseSiteNullChecks` is passed, the compiler won't generate them

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Commit messages:
 - generate by default
 - add use site null checks

Changes: https://git.openjdk.org/valhalla/pull/1910/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1910&range=00
  Stats: 155 lines in 2 files changed: 147 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/valhalla/pull/1910.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1910/head:pull/1910

PR: https://git.openjdk.org/valhalla/pull/1910