Make UnusedVariable catch (some) unused assignments, e.g.:

  int a = 1;
  ...
  a = 2;
  return a;

This doesn't do anything really fancy like detect:
  int a = 1;
  if (hashCode() > 0) {
    a = 2;
  } else {
    a = 3;
  }
  return a;

RELNOTES: Make UnusedVariable match unused assignments too.

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