Do actual DCE in LoopUnroll (try 2)

Recommitting after addressing a missed review comment, and updating an aarch64 test I'd missed.

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