Teach jump threading to clean up after itself, DCE and constfolding the

Teach jump threading to clean up after itself, DCE and constfolding the
new instructions it simplifies.  Because we're threading jumps on edges
with constants coming in from PHI's, we inherently are exposing a lot more
constants to the new block.  Folding them and deleting dead conditions
allows the cost model in jump threading to be more accurate as it iterates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60327 91177308-0d34-0410-b5e6-96231b3b80d8