RFR: fixing bugs in NullChecksWriter

[Vicente Romero]

fixing some bugs in NullChecksWriter, reported by John Rose. In particular there was no null check being generated by javac for code like:


class Test {
    void m(String someObject) {
        String! x = "foo";
        x = (String)someObject;  // no null check generated here
    }

    public static void main(String... args) {
        new Test().m(null);
    }
}

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

Commit messages:
 - removing dead code
 - fixing bugs in NullChecksWriter

Changes: https://git.openjdk.org/valhalla/pull/1883/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1883&range=00
  Stats: 26 lines in 2 files changed: 11 ins; 15 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1883.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1883/head:pull/1883

PR: https://git.openjdk.org/valhalla/pull/1883