RFR: add different flavors to the useSiteNullChecks compiler option [v2]

[Vicente Romero]

> Changing the option to indicate if use site null checks should be emitted for fields and methods. The option has be renamed to: `useSiteNullChecks` with sub-options to indicate if no use site checks should be generated, or if it should be generated only for methods or for methods and fields. The default is generating the checks for fields and methods

Vicente Romero has updated the pull request incrementally with two additional commits since the last revision:

 - removing unused variable
 - addressing review comments

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1939/files
  - new: https://git.openjdk.org/valhalla/pull/1939/files/c3fbd863..f804e6d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1939&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1939&range=00-01

  Stats: 38 lines in 1 file changed: 23 ins; 13 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1939.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1939/head:pull/1939

PR: https://git.openjdk.org/valhalla/pull/1939