DeadCodeElimination: rewrite to be faster

DeadCodeElimination: rewrite to be faster

Same strategy as simplifyInstructionsInBlock. ~1/3 less time
on my test suite. This pass doesn't have many in-tree users,
but getting rid of an O(N^2) worst case and making it cleaner
should at least make it a viable alternative to ADCE, since
it's now consistently somewhat faster.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248927 91177308-0d34-0410-b5e6-96231b3b80d8