Revert "[DSE] Switch to MemorySSA-backed DSE by default."

There appears to be a mis-compile with MemorySSA-backed DSE in
combination with llvm.lifetime.end. It currently appears like
DSE is doing the right thing and the llvm.lifetime.end markers
are incorrect. The reverted patch uncovers the mis-compile.

This patch temporarily switches back to the legacy DSE
implementation, while we investigate.

This reverts commit 9d172c8e.