Fix PR17952.

  Fix PR17952.

  The symptom is that an assertion is triggered. The assertion was added by
me to detect the situation when value is propagated from dead blocks.
(We can certainly get rid of assertion; it is safe to do so, because propagating
 value from dead block to alive join node is certainly ok.)

  The root cause of this bug is : edge-splitting is conducted on the fly,
the edge being split could be a dead edge, therefore the block that
split the critial edge needs to be flagged "dead" as well.

  There are 3 ways to fix this bug:
  1) Get rid of the assertion as I mentioned eariler
  2) When an dead edge is split, flag the inserted block "dead".
  3) proactively split the critical edges connecting dead and live blocks when
     new dead blocks are revealed.

  This fix go for 3) with additional 2 LOC.

  Testing case was added by Rafael the other day.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194424 91177308-0d34-0410-b5e6-96231b3b80d8