* src/hydrogen-instructions.cc (HValue::InferRange): Only mark values
  with int32 representation as never being -0.  Always return a non-NULL
  value; callers should check for representation().IsNone() if that's
  their concern.

  In practice these invariants were not violated by callers, but they
  were sometimes two calls away, which seems brittle.

BUG=
TEST=tests pass, modulo http://code.google.com/p/v8/issues/detail?id=1572

Review URL: http://codereview.chromium.org/7514040
Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8804 ce2b1a6d-e550-0410-aec6-3dcde31c8c00