Do actual DCE in LoopUnroll (try 3)

Recommitting after fixing a bug found post commit.  Amusingly, try 1 had been correct, and by reverting to incorporate last minute review feedback, I introduce the bug.  Oops.  :)

The problem was that recursively deleting an instruction can delete instructions beyond the current iterator (via a dead phi), thus invalidating iteration.  Test case added in LoopUnroll/dce.ll to cover this case.

LoopUnroll does a limited DCE pass after unrolling, but if you have a chain of dead instructions, it only deletes the last one. Improve the code to recursively delete all trivially dead instructions.

Differential Revision: https://reviews.llvm.org/D102511