Quick: Fix marking high words in DCE.

This CL properly fixes the high word marking and reverts
    https://android-review.googlesource.com/150352
which was just covering up the underlying issue. A unit test
for the encountered issue is provided, though it does not
expose the deficiency in the cover-up CL.

Bug: 20640451
Change-Id: Ia19a1134610e271ebbab5679ac0bd0cb0223d462