RFR: add use site null checks [v9]

[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 two additional commits since the last revision:

 - Apply suggestion from @vicente-romero-oracle
 - last change

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1910/files
  - new: https://git.openjdk.org/valhalla/pull/1910/files/7fc61058..51bb31c6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1910&range=08
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1910&range=07-08

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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