1) have it fold "br undef", which does occur with
surprising frequency as jump threading iterates.
2) teach j-t to delete dead blocks. This removes the successor
edges, reducing the in-edges of other blocks, allowing
recursive simplification.
3) Fold things like:
br COND, BBX, BBY
BBX:
br COND, BBZ, BBW
which also happens because jump threading iterates.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60470 91177308-0d34-0410-b5e6-96231b3b80d8
