Fixed bug in BCE, with regression test.

The fix is twofold:
(1) Ensure that bound checks are never eliminated more than once
    to guard against any conceivable situation in which the same
    bounds check appears multiple times in array length use list.
(2) Specially reject BoundsCheck(x,x) since that always goes OOB.

BUG=27628526

Change-Id: I399ec4254323e0cfcd0a68898f403cfab7b35135