Revert "Revert "Null check elimination improvement""

This reverts commit 31aa97cf.

..and thereby brings back change 380165, which was reverted
because it was buggy.

Three problems with the original CL:

  1.  The author ran the pre-submit tests, but used -j24 and
      failed to search the output for fail messages.
  2.  The new null check analysis pass uses an interative
      approach to identify whether a null check is needed.  It
      is possible that the null-check-required state may
      oscillate, and a logic error caused it to stick in the
      "no check needed" state.
  3.  Our old nemesis Dalvik untyped constants, in which 0 values
      can be used both as object reference and non-object references.
      This CL conservatively treats all CONST definitions as
      potential object definitions for the purposes of null
      check elimination.

Change-Id: I3c1744e44318276e42989502a314585e56ac57a0