There are cases when C2 fails to remove redundant check (has identical dominating check).

Reported by Vitaly Davidovich:
  http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2015-November/019930.html

Final IR from attached test case:
RedundantNullCheck::doIt() {
  w = Load this._w
  If (w == NULL) deopt
  i = Load w._i
  If (i != NULL) { // check #1
    If (!i instanceof C) deopt
  } v1 = Phi(T:1,F:-1);
  If (i != NULL) { // check #2
    If (!i instanceof C) deopt
  } v2 = Phi(T:2,F:-1)
  return v1+v2
}

Checks #1 & #2 are identical. #1 dominates #2, but C2 doesn't remove #2.

Next transformations should be:
  w = Load this._w
  If (w == NULL) deopt
  i = Load w._i
  If (i != NULL) {
    If (!i instanceof C) deopt
    If (!i instanceof C) deopt
  } v1 = Phi(T:1,F:-1)
    v2 = Phi(T:2,F:-1)
  return v1+v2

and

  w = Load this._w
  If (w == NULL) deopt
  i = Load w._i
  If (i != NULL) {
    If (!i instanceof C) deopt
  } v1 = Phi(T:1,F:-1)
    v2 = Phi(T:2,F:-1)
  return i1+i2;