RFR: add use site null checks for fields [v2]

[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

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

  updated code

-------------

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1926/files
  - new: https://git.openjdk.org/valhalla/pull/1926/files/895ba42a..0fd2f2d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1926&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1926&range=00-01

  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 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