RFR: add use site null checks [v6]

[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

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  code changes

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1910/files
  - new: https://git.openjdk.org/valhalla/pull/1910/files/06453d99..ece0ee79

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1910&range=05
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1910&range=04-05

  Stats: 9 lines in 1 file changed: 2 ins; 3 del; 4 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